# Core API concepts

- [Authentication](/docs/get-started/core-api-concepts/authentication.md)
- [External identifiers](/docs/get-started/core-api-concepts/external-identifiers.md)
- [Date and time formats](/docs/get-started/core-api-concepts/date-format.md)
- [Requests and filters](/docs/get-started/core-api-concepts/requests.md)
- [Pagination](/docs/get-started/core-api-concepts/pagination.md)
- [HTTP status codes](/docs/get-started/core-api-concepts/http-status-codes.md)
- [Rate limiting](/docs/get-started/core-api-concepts/http-status-codes-1.md)
- [Send documents](/docs/get-started/core-api-concepts/send-documents.md)