Internal I.D.

Hi all can someone point me to where I can find some detailed information describing this elusive Internal I.D. I've always been hearing about it in SolidWorks and I have a rudimentary idea of it and the concept, but I can't find a detailed description of nor the list of rules to know and follow regarding Internal I.D.'s

Thanks, Daniel

P.S. Roland I wasn't aware of that.
SolidworksGeneral