I am sure I have used it somewhere but for the life of me cannot remember where but vaguely recall this situation....
Passed BO to a process input is instance of MyBO (This)
FIND MyBO WHERE Blah, Blah (some other instance of MyBO..... That)
ExecuteMySubProcess USING ThatMyBO
The other place which I discovered some time ago..... slightly diferent but neat all the same....
Imagine you have a Query of Child and the Parent is not in context. But, the Child is Owned by Parent.
When calling a process from say, Operation With Reocrds or Panel Operations you can pass a Reference.... Expected By Process Parent , Passed from Operation Child.ob_Parent
If you did not know that then you would probably set the Process to have Child as the Input and have FIND Parent WHERE Parent = Child.ob_Parent as the first Rule in the Process but it is not necessary.