An in-line sub process can best be thought of as a callable scope. It is a scope that is not part of the normal process flow, but rather a unit of encapsulated logic that can be called zero, one or multiple times from anywhere within the BPEL process – even recursively from within itself. It is in effect very similar to a private member function in a Java Class or a procedure in the body of a PL/SQL package. Just like private member methods or functions help structuring a Java Class or PL/SQL package, so can subprocesses help organize a large BPEL process – even more than scopes already can.
An in-line sub process can define arguments to be passed in. These can be passed by reference, which means the sub process can alter their values as well. Additionally, the in-line sub process can access all global process variables. The in-line sub process is a scope which as such can define local variables and nested scopes, use partner links, have event handlers etcetera. At run-time, the subprocess code is re-entrant and re-usable – only one copy is stored in memory, even if it is called multiple times. However, on each call, it has a private scope for local and passed-by-value variables.