Hi there,
your question is a really good question. My personal answer is: it depends.
It really depends on how you are planning to use jBPM5. The project let's you choose about how to manage the relationship between your processes and your sessions. For example:
You can create a session per user and handle it's internal workflows or you can create a big session that handle a lot of work.
You can keep track of the session that it's handling the process in a very simple map.
Cheers