Exoneration means you broke a rule but are not penalized and need not take a penalty.
43.1
(a) If another boat's breach compelled you to break a rule, you are exonerated. A chain collision where the boat behind was pushed into you is the classic case.
(b) If you are sailing within room or mark-room you are entitled to and, because the boat that owed it failed to give it, you break a Section A rule, rule rule 15, rule 16 or rule 31, you are exonerated.
(c) A right-of-way boat, or one sailing within the room or mark-room she is owed, is exonerated for breaking rule rule 14 if the contact causes no damage or injury.
43.2
An exonerated boat need not take a penalty and cannot be penalized for that breach.
Common misreadings
"Rule 19.2(a) is covered by 43.1(b)"
It is not. A right-of-way boat that fails to give room to keep clear when choosing her side at an obstruction is not exonerated, unlike a breach of rule 16.