Collisions between the pair of bodies is never ok, i.e., if two bodies are in contact in a particular configuration of the robot, that configuration is considered to be in collision.
ALWAYS
Collisions between a particular pair of bodies does not imply that the robot configuration is in collision. There is no need to explicitly do a computation (to check for contacts) on this pair of bodies.
CONDITIONAL
The collision is allowed depending on a predicate evaluated on the produced contact. If the predicate returns true, this particular contact is deemed ok (or allowed), i.e., the contact does not imply that the two bodies are in collision.