Q. Criteria for full sync

I have 3 nodes replica set structure of P-S-S.
But one secondary was down for some reason, and it’s been a long time.
There has been a lot of data coming in from Primary, and I start Secondary again.
At this time, this secondary became full sync, and I want to know about the criteria for executing full sync.

My think : If primary’s oplog runs one round, secondary’s oplog fails to catch up, execute full sync.

Is there any other case?