I thought this was a silly question, as we all "know" what we mean by a Randonnée. But it isn't that clear. If you look at a Calendar event, the Category is clearly listed as BR or BRM which count for RRTY. But for Perm events, the Category is listed as Permanent.
The Perm has got to be 200km or more at a speed of 14.3kph. There are lots of BP Perms at distances up to 199km.
The real clue is whether it has any points against it on your Individual Rider List, but that's rather after the fact.