As Michael said it's the "ordinality" of the workunits, meaning the fist unit sent out ends in a _0, the 2nd task sent out ends in _1, the 3rd task sent out ends in _2 etc, etc. This has to do with whether a task is being resent to other users because the first user had problems with the workunit or for whatever other reason, timed out etc etc. So for example a workunit with the last digits being a _7 would mean it's the 8th time that particular workunit is being sent out to get crunched.
You can see when looking at a workunit that most units are only sent out one time:
minimum quorum 1
initial replication 1
max # of error/total/success tasks 15, 15, 5
But they can be sent out as many as 15 times if needed before the project itself steps in and tries to figure out what's wrong with the task.