# External identifiers

Use external identifier operations to associate entities such as customers,
invoices, transactions, journal accounts, journal entries,
and more with external services.

To use external identifiers, you must use an active service credential.

## Retrieve external identifiers

 - [GET /external-identifiers](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/getexternalidentifiercollection.md): Retrieves a list of external identifiers.

## Retrieve an external identifier

 - [GET /{resource}/{resourceId}/external-identifiers/{service}](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/getexternalidentifier.md): Retrieves an external identifier with a specified resource, resource ID and, service name.

## Sync an external identifier

 - [POST /{resource}/{resourceId}/external-identifiers/{service}](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/postexternalidentifier.md): Synchronizes an external identifier with a specified resource, resource ID, and service name.

## Upsert an external identifier

 - [PUT /{resource}/{resourceId}/external-identifiers/{service}](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/putexternalidentifier.md): Creates or updates an external identifier with a specified resource, resource ID, and service name.

## Delete an external identifier

 - [DELETE /{resource}/{resourceId}/external-identifiers/{service}](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/deleteexternalidentifier.md): Deletes an external identifier with a specified resource, resource ID, and service name.

## Retrieve external service settings

 - [GET /external-services-settings](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/getexternalservicesettings.md): Retrieves external service settings.

## Update external service settings

 - [PUT /external-services-settings](https://www.rebilly.com/static/_spec/catalog/all-3.0/external-identifiers/putexternalservicesettings.md): Updates the external service settings.