Once again like many archery topics, the terminology can be different depending on how you're describing it.

An arm can have a sliiiiight bend to it, yet still be in a good bone-on-bone form. Everybody's arms are a bit different, and have slightly different makeup structure.

A straight-out hyper-extended arm is no good either. And an arm that is too bent can be very inconsistent and hard to reproduce the same way every shot. Anything that depends on muscles to hold form is a thing that leads to inconsistencies in form and shot execution. Ideally (IMO) the only muscles you really want to use are the ones required to keep the bow up and on target, and muscles to keep the bow at full draw while aiming and then start applying a good backtension for a clean release.

The more muscles you involve in shot execution, the more inconsistent you'll be. Of course that's a general statement and there's always exceptions. I've some very good shooting from guys who's form was so whacked that I would swear would be all over the place. But that rule generally seems to hold water. Bone-on-bone contact is fairly consistent, your bones don't change size from shot to shot and it's much easier to reproduce bone on bone placement. Muscles holding a position (like a really bent bow arm) are inconsisent, and much more prone to fatigue and form deterioration over time.