> ## Documentation Index > Fetch the complete documentation index at: https://docs.arcuserp.com/llms.txt > Use this file to discover all available pages before exploring further. # Apply a payment to multiple invoices > Creates one payment and applies it across the supplied invoices, or, with `auto_apply`, allocates it oldest-first across the account's open invoices, posting one journal entry for the applied portion. A surplus over what the open invoices can absorb is refused unless `route_unapplied` is set to `credit`, which holds the extra as a credit on the account instead of silently capping it; idempotent via the `Idempotency-Key` header. ## OpenAPI ````yaml /openapi.yaml post /payments/multi-invoice-apply openapi: 3.1.0 info: title: Arcus ERP Public API version: 1.0.0 description: > Arcus ERP public REST API. Designed for external integrations and data migration. **Authentication.** Bearer token (API key) via the `Authorization` header. Format: `Authorization: Bearer ark_live_ent__` (or `ark_test_*` for sandbox). API keys are issued per-entity in **Settings > Developers > API Keys**. **Entity scoping.** The entity is encoded in the API key prefix; routes are flat (e.g. `/v1/accounts`, `/v1/orders`, `/v1/products`). A small set of platform endpoints (migration, reconciliation, events, webhook endpoints, API keys) use the `/v1/entities/{entity_id}/...` form -- those are noted in their tags. **Key capabilities.** - Related-resource hydration via `?expand[]=` (see `x-arcus-expand` on each resource). - Cursor-based pagination (`starting_after` / `ending_before` / `limit`). - Idempotency via the `Idempotency-Key` header. - Webhook events for asynchronous notification. - Conditional requests / ETag for cache validation. **Changelog.** Entries are dated and name every published contract whose MEANING moved, not only the ones whose field names changed. The narrative version of the same entries, written for integrators, is published at https://arcuserp.mintlify.app/changelog. **2026-09-22 (planned 2026-09-21), REORDER-BUYER-TRUTH: demand changed what it MEANS on three published contracts, with no field renamed.** An integrator that pins field names sees no breakage and different numbers, which is why this entry exists. - **Demand now counts build consumption.** `demand_avg_per_day` on the public product, kit and inventory-balance objects, and `daily_demand` / `demand_basis` / `net_suggested_qty` on `GET /v1/purchasing/reorder-report`, are composed from fulfilled sales lines PLUS posted `build_consume` inventory draws: each physical decrement of a product counts exactly once. The previous rule adopted an internal-consumption basis only when the sales blend was exactly zero, so a product both sold AND consumed into work orders planned as if the build draws did not exist. Products drawn into work orders move; on one production-shaped dataset five did, the largest from 0.05/day to 14.95/day. - **`demand_basis` now carries four values, not two:** `sales`, `sales_and_builds`, `builds` and `consumption`. A consumer with a two-branch reader (anything that is not `consumption` is `sales`) silently hides the two new ones. - **`current_demand_units` on `GET /v1/inventory/balances/:id` moved, by a second rule.** It counts committed-but-unshipped CUSTOMER demand, and it now counts a kit component's own line rather than its parent kit line, and excludes non-sales documents. On one production dataset 269 of 1,043 balance rows changed, 227 of them downward; the largest single move was 1,941 to 25, on a product whose open PURCHASE order line had been reported as customer demand. Re-baseline anything that alerts or reorders off this field. - `run_rate_30/90/180/365` and `blended_daily` on the same balance object move for the first reason above. - **Added, not changed:** `GET /v1/purchasing/reorder-report` accepts `demand_window` and returns the unit, cover-through and reorder-point-provenance fields documented on that operation; `order_multiple` is accepted and returned on the product-vendor doors. - **Volume note.** The `inventory.low_stock`, `inventory.out_of_stock` and `inventory.back_in_stock` webhook events are population-gated on `on_hand <= reorder_point`, and reorder points move with the demand above, so subscribers should expect a one-time step change in event volume around the release. servers: - url: https://api.arcuserp.com/v1 description: Arcus ERP API (accepts both live `ark_live_*` and test `ark_test_*` keys) - url: https://dev-api.arcuserp.com/v1 description: >- Dev sandbox API (test-only data, accepts `ark_test_*` keys against dev RDS) security: [] paths: /payments/multi-invoice-apply: post: tags: - Payments summary: Apply a payment to multiple invoices description: >- Creates one payment and applies it across the supplied invoices, or, with `auto_apply`, allocates it oldest-first across the account's open invoices, posting one journal entry for the applied portion. A surplus over what the open invoices can absorb is refused unless `route_unapplied` is set to `credit`, which holds the extra as a credit on the account instead of silently capping it; idempotent via the `Idempotency-Key` header. operationId: applyPaymentToInvoices parameters: - $ref: '#/components/parameters/IdempotencyKey' requestBody: required: true content: application/json: schema: type: object required: - account_id - payment_method - amount properties: account_id: type: string format: uuid description: The customer account receiving this payment. payment_method: type: string enum: - check - ach - wire - external - cash - credit description: Tender method for this payment. amount: type: number description: >- Total payment amount in dollars (cash actually received). The sum of applications[].amount_applied must be <= amount; any surplus requires route_unapplied='credit'. example: 5000 auto_apply: type: boolean description: >- When true (and applications omitted), the server allocates the payment oldest-first across the account open invoices. The response returns the computed applications[]. example: true route_unapplied: type: string enum: - credit description: >- When 'credit', any surplus (amount beyond what open invoices absorb) is held as a credit-on-account via a balanced journal entry in the same transaction. Without it, a surplus is refused (no silent cap). applications: type: array minItems: 1 maxItems: 50 description: >- Per-invoice application lines (manual override). Omit when auto_apply=true. Sum of amount_applied must be <= amount; any remainder requires route_unapplied='credit'. items: type: object required: - order_id - amount_applied properties: order_id: type: string format: uuid description: >- Invoice (order) to apply this portion of the payment to. amount_applied: type: number description: >- Dollar amount to apply to this invoice. Must be > 0 and <= invoice balance_due. example: 2500 check_number: type: string description: Check number (for check payments). check_date: type: string format: date description: Check date (YYYY-MM-DD). external_reference: type: string description: External reference (ACH trace number, wire memo, etc.). notes: type: string description: Internal notes for this payment. payment_date: type: string format: date description: Payment date for GL posting (YYYY-MM-DD; defaults to today). responses: '200': description: Payment applied successfully content: application/json: schema: type: object properties: data: type: object properties: id: type: string format: uuid entity_id: type: string format: uuid amount: type: number payment_method: type: string status: type: string journal_entry_id: type: string format: uuid applied_count: type: integer applications: type: array items: type: object properties: id: type: string format: uuid order_id: type: string format: uuid amount_applied: type: number idempotent_replay: type: boolean '400': $ref: '#/components/responses/Error' '401': $ref: '#/components/responses/Error' '403': $ref: '#/components/responses/Error' '422': $ref: '#/components/responses/Error' security: - ApiKeyAuth: [] components: parameters: IdempotencyKey: name: Idempotency-Key in: header required: false description: | Client-generated unique key for idempotent POST/PATCH/DELETE operations. Alias for the Idempotency parameter. Max 255 chars. On retry with the same key, the original response is returned without re-executing the operation. Keys expire after 24 hours. schema: type: string maxLength: 255 responses: Error: description: Error response (400/401/403/404/409/422/429/500) content: application/json: schema: $ref: '#/components/schemas/ErrorEnvelope' schemas: ErrorEnvelope: type: object description: | Canonical error response envelope. All API errors use this shape. required: - error - code properties: error: type: string description: Machine-readable error key example: not_found code: type: string description: Machine-readable error code (often same as error) example: not_found type: type: string enum: - validation_error - permission_error - not_found - conflict - rate_limit - internal - expand_error - not_implemented example: not_found hint: type: string description: Human-readable one-sentence explanation (English) example: >- The requested order does not exist or does not belong to this entity. param: type: string description: The parameter that caused the error, if applicable example: expand[0] required: type: string description: The scope required (only on insufficient_scope errors) example: accounts:read request_id: type: string description: >- Unique request ID for support tracing (maps to CloudWatch log stream) example: req_abc123 securitySchemes: ApiKeyAuth: type: http scheme: bearer description: | API key issued per entity via Settings > Developers > API Keys. Each key carries scopes (e.g. orders:read, products:write). Bearer token format: Authorization: Bearer ark_live_ent__ Test keys use ark_test_ent__. Both are issued per entity via Settings > Developers > API Keys. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.