In a service-oriented architecture business software implementation, the Enterprise Service Layer (ESL) is the highest level of abstraction.
Any application programming interface (API) defined at the ESL can cross domain boundaries; it calls directly the Domain Service Layer, which in turn interacts with the Application Service Layer or the RDBMS Service Layer. Therefore, any API which must access multiple domains to execute correctly must exist at the enterprise level.
Since the ESL is the API of the entire enterprise, all the components in the enterprise can call it directly, and it can sometimes be accessed from outside the service-providing entity.
ESL exposes a number of API considerably lower than the ASL because it works at a higher level of abstraction than the ASL.