The saying goes with paint correction, if you cant feel the scratch with you finger nail then it can be corrected (usually), have a look about on paint correction theres loads infomation on tinternet!
Very true, as it would have gone through the clearcoat in most instances, so there is no 'room' for correction
Correction is simply cutting back the clearcoat slightly to remove the defects which you can see (scuffs, holograms, swirls etc)