Model Process Status - STAT_UNKNOWN

I’m curious to know as to what are the possible process status PRT stores in the database. For ex: STAT_RUNNING, STAT_UNKNOWN.

I’m interested particularly about STAT_UNKNOWN. This is because one of the process steps in my model seem to have problem and doesn’t get executed sporaidically. When I increased the debug level and checked the server log, the only difference I could find is the process status. For all other instances, the process step says STAT_RUNNING. However for the instance under discussion, the server log has recorded STAT_UNKNOWN.

SIZE=2 checking process status
(T10) status = STAT_UNKNOWN
(T10) step iteration = 1
(T10) evaluating join
Thanks for your time,
Sivaraj Lenin