OP - there is a lip cut at end of your chamber - gives room for neck behind and naked bullet ahead of that. "Case Length" is that you do not want your case to be longer than that lip - either when reloaded or when firing - if your case neck extends forward of that lip, it will pinch tighter on the bullet - harder to let go - changes your breech pressure - so "trim length" was supposed to be established to prevent your loaded round or your "while firing" round to be able to grow that much. So far as I know, you could trim your brass a 1/4" shorter than recommended - will work fine, although you will likely loose a lot of "neck tension" by doing so.
Your 7mm Rem Mag was supposed to be set up to headspace on that rim that was cut near the rear of your chamber - is what that belt on your case does - prevents that case from going into chamber too deeply. Some will suggest to transfer your "headspace" (endplay) from that belt to the shoulder of your chamber - to make your case a snugger fit in your chamber. At same time, you could probably play with case neck length - maybe your rifle shoots better with much more trim than recommended - is almost all what you get on targets - bullet holes do not lie. They will tell you whether your change was an improvement, made no difference or made things worse. So far as I know, you pretty much have to shoot targets to know, for your loads - how you put them together - when fired in your rifle.
