A SEP is a container inside which a CICS transaction or a JES program runs.
In Enterprise Server for .NET, SEPs operate as discrete application domains rather than as processes, to keep SEPs isolated.
SEPs are hosted within a SEP pool.
A SEP pool is a collection of AppDomains.
Each SEP is responsible for the execution of a single request. When a request completes, the SEP is returned to the pool and made available for a subsequent request execution.
SEPs are removed from the pool if the SEP needs to be abended due to a timeout, or if the associated terminal disconnects.
SEPs can be self-hosted in seesep.exe processes, or WAS-hosted in w3wp.exe processes. WAS-hosted SEP processes are started on demand and periodically recycled by WAS.