I am a colleague of Darren's and can add a bit more detail:
My suspicion is that this might be caused by the property wt.team.teamChunkSize (or the chunk size site level preference) which is set to 1000 on our System (as directed by the Windchill Archive/Purge Guide) - I suspect that it tries to purge items in groups of 1000 and if there are referential items outside of this group of 1000, then the related item in the chunk is not allowed to be purged. When you then run the same Purge rule again, there are differing items in each chunk and some more items are able to be purged each time. The trouble is that I have run the same purge rule 8 times now and it is still finding data to purge each time!!
I would not expect this behaviour, though - my understanding of purge is that you should only need to run it once and it should purge everything as per the purge rule specification (but in chunks).
Unfortunately setting wt.team.teamChunkSize back to -1 (unlimited) is not possible as then the purges fail because they hit the search query limit - even small purges seem to fall foul of this and I have boosted it from the recommended 350,000 to 1,000,000 to see if I can get a purge to complete.
Does anyone have an explanation for what we are seeing? Is it expected behaviour, or is something wrong here? How do we go about sensibly being able to purge data from our system (with Chunk Size set) without running a purge on a product context many many times?
Rgds
Gary