Optional processes are usually multi-step processes that are outside the primary path of the current case. An optional process would allow the user to run a new process from within the case at their discretion (any point during the case). For example, I might have a loan processing application. As we get further into the process we notice that the interest rate that is going to be charged it very high. This is due to the applicants poor credit history.
We might have a process that would automatically create new cases (leads) for a third-party credit counseling service that could help the client improve their credit score and reapply as a later date. The optional process would not have any affect on the loan process.