Let's say you have two processes named "parent" and "sub." Both are version 1.0. Parent passes one parameter to sub, and sub returns one parameter to parent.
Next, you initiate a few instances of parent.
Next, you create and activate a version 1.1 of both parent and sub, and you modify the parameters passed between the two.
The parent instances created when the version was 1.0 are still running. During their execution, they invoke sub.
Will the invokation of process sub be version 1.0 or version 1.1?
It'll always call the head revision of a process.