Benny,
Hopfully Rocky will respond to your question. He is a pro @ descirbing simple but complicated things.
If you're speaking of the 7mm REM, the cartridge head spaces on the belt. Beltless cartridges head space on the shoulder.
It all has to do with case stretching when shooting. Sizeing too much creates too much head space. And sizing too little, not bumping the shoulder back once in awhile creates too little head space. Or is it the other way around.
Like I said, Rocky's the the man to answer.