Cancel Marks

I recently saw a comment when searching online old stamps. And in doing so I saw something which made me stop and think about catalogues when they say used or mint.

So the question is, does a First Day of Issue cancel mark include as used in the correct sense? Or better still what constitutes as a cancel nark in the sense used to Catalogues?

I have never realised just how many questions can be made upon such a single thing on a cover! In a discussion we had in here some time back about "cancels" "do they make a cover more valuable", there would now be a question whether this is true or not? I think this is when you consider Stanley Gibbons specialized catalogue, they do give a picture of cancels on covers and a price from recollection but I cannot remember if the cancel was given as a single component outside of the cover intended to have the cancel, I believe SG only do the value for a cancel in a set of one issue meant for that cover. I'm struggling to explain my question but I have misplaced my catalogue to give a for instance.