It depends what version of CTD you have really, there are a lot of variations. But to really simplify it the internal pressure charge has either leaked into the oil, out somewhere else, or was never really there to start with. This means there isn’t as much pressure pushing back on the valves so the ‘propedal’ forces are weaker. If it’s a boost valve shock you will actually see the tune number printed on the air can, usually 175-250ish. Higher then number, firmer the lever makes it. Sort of.
Air can service is just lubing the air sleeve, full service is the damper too, so the bit that controls the compression, rebound and lockout.
If you want to see the difference go on the fox site, full service guides are there so you can see inside your shock. Don’t pull it to bits though, as you need a bit of kit to put it back together. Get it off to TF or similar