Hi Matthew, as most of us learn, a relatively new mattress is essentially flat when in situ and when we lay on it. However as it gets use it forms what could be called permanent dishes ( troughs, sag ) where the bodies lay.
Using a double / queen / king as example, some mattresses will become something like this when viewed side to side -
whether or not one is on the mattress, and worse when on. The central "hill" (or sag from level) can be up to 40-50mm according to some warranties. A mattress that provides comfort and good service should not develop much sag, nor should that "hill" (or "sag" from level) be more than 10-15mm from "level" in my opinion.
Looking from head to foot a mattress is supposed to conform to the body, but many will soon develop permanent sags where the heaviest part of the body lies.
Slightly edited for the purposes here, from a warranty overview on "sleeplikethedead.com" ->
...The main complaint people have with their mattress is sagging....
Owner experience data suggests that even moderate sagging can and often does result in less comfort and support. It may also cause back pain, especially for side and stomach sleepers. ...
.... Most warranties will cover sagging only when the problem becomes severe enough, that is, when it reaches a certain depth.
For example, [some] mattress warranties will often provide coverage for sagging beginning at a depth of 1.5 inches. By contrast, [others] provide coverage for sagging beginning at a depth of .75 inches. ....