After analyzing a game or a match GNU Backgammon is able to do some comments on checker play or cube decisions. Open the menu `Window, Game record' and you will see a list of the moves. If the actual move or cube decision differs from the best, GNU Backgammon will put a comment on it.
The default settings are:
You may change these comment in the menu `Window, Annotation' choosing another comment for moves and/or double
Why a ? move should be deemed worse than a ?! move - that's one of life's little mysteries.