# External IDs

Every organization and invoice is identified by an `externalId`, the identifier from your own
system. You do not store Lunch identifiers.

## Invoice Identifiers

Invoice identifiers are unique per payee, not per account. For this reason:

- The invoice sync body includes both the `payee` and the `payor`.
- `POST /v1/advances` requires `payeeExternalId` and `invoiceExternalId`.

## Unrecognized Identifiers

Invitations and quotes return `404 business_not_found` for an organization that has not been synced.
Nothing is created. Syncing an organization, or an invoice that names it, makes the identifier
available.

## Resending Records

Writes are idempotent, so you do not need to track what you have sent. Resend records when they
change or on a schedule.

## What's Next

- [Idempotent Requests](/idempotent-requests)