It only models what it is given. Whoever did the cell library chose
a range of risetimes and loads to characterize and build the
timing model tables (or functions) from. This is your "known space".
Complementary to this are a set of hard constraints, the "fence" you
can't go outside and believe the STA timing closure.
What STA (static) can't deal with, is dynamic effects such as the
next wire over, which STA treats as a simple shunt capacitance,
switching at exactly the wrong time and advancing a clock
or retarding data or creating a timing hazard / spike where there
would have only been slowness.
If you work with comparators you know about how a slow input
risetime enables output chatter based on small amounts of
input noise. Your "chatter window" is the transform of input
signal slope and input "linear window" (roughly (VOH-VOL)/gain).