What a reference is actually asked

Many people picture a reference as a formality — dates confirmed, a box ticked. Sometimes it is. Often it is a phone call in which somebody asks what you were like to work with, how you handled something going wrong, whether they would take you back, and whether there is anything they think should be known.

That last question is the one that matters, and it is why the choice of referee is not a formality either. A referee who likes you but cannot recall anything specific will answer in generalities — and generalities from a referee are read as reservation.