JoinNotFound

I have seen a post on here with the same error but it didn't seem to help me so rather than hijack the thread I'd ask again. More specifically, I don't understand which attribute it's trying to match on and how to set/check that.

I get this when doing a provision on demand.

"No action required. User '....' is not a newly discovered entry to be provisioned in the target application, nor one with an update that should flow to a target entry with which it was previously matched."

Skipreason: JoinNotFound.

I'll go through the documents again on Microsoft Learn in case I've missed something but if anyone has any pointers I'd be most grateful.

TIA

Andrew