My long answer....
Full custom (in my opinion) includes custom everything, such as:
Stiller TAC Action
Stiller CIP DBM
Proof Carbon Barrel
Terminator T3 brake
Manners EH1-A in Elite GAP camo
Triggertech Diamond
Where as a "semi custom" could have all the same components, except a Trued Remington 700 action (or other production action). The action very likely will not be as nice as the custom aftermarket actions such as stated above, unless a LOT of work is done to them. Work such as a good true job, properly timed fluted bolt with m16 or similar extractor, bolt is fitted or sleeved to the action, firing pin has been bushed, #8 screws and a pinned rail, pinned recoil lug, side bolt release, extended ejection port, and opened up to accept extended bottom metal. This is pretty much all stuff that is considered "standard" on a custom action, and generally does not need to be done to any of the customs. So if that work wasn't done to your action, then I PERSONALLY don't feel like it is actually a "full" custom. And even if it was, it is still only modifications made to a production action.
My rifles are mostly semi customs. Remington actions that have extended bolt knobs and true jobs, possibly bolt fluting and an extractor installed, and that is about it. Everything else aftermarket. Not saying that they can't be as accurate as full customs, but simply put, they aren't as nice. My primers on my rem 700 .260 AI are significantly cratered even with low pressure loads due to a very sloppy firing pin hole that needs bushed, for example. Once I get that fixed, I will have about as much $$ into that action as I would have on an entry level custom action. On the Stiller build I put together, nothing like that was present, and it was much smoother overall, nothing needed done.
My short answer....
This is mostly semantics, but in my opinion, a "Full Custom" is exactly that, EVERY component is a FULL custom part, not a modified production part. Everything else falls into a much wider range, from someone that has simply replaced a trigger to everything is aftermarket except the action.