what exactly do COL's get granted for?

Can anyone tell me what a COL [certificate of lawfulness] can be applied for and used for?
Ive just found one being granted for a garage conversion [from a garage to a kitchen/diner] The planning officer has granted it a COL because apparently you do not need planning permission to do this type of conversion.
I thought that if you were simply replacing a garage door with a window and turning that area into a kitchen you would have to seek planning permission, not seek a COL for it?
What exactly does a 'certificate of lawfulness' mean when applied for like this, I have always thought that they only applied to existing bedsit units, do I take it that they cover other types of conversions too?