isExclusive is for the following purposes:
suppose you have a fork and right after the fork you have asynchronous continuations on all the concurrent paths of execution.
that means that all the jobs on the same process instance will be scheduled to be executed around the same time. the transactions in which the continuation will take place, will have a high probability of generating optimistic locking failures (update of global process variables or joining). So in that case it is not really usefull to have all those asynchronous continuations to be executed concurrently.
so all exclusive jobs for a single process instance should be locked and executed by the same job executor thread so that they can't generate optimistic locking failures.