parasitic extractions cworst

rcworst gives worst [max] delay? rcbest gives best [min] delay? so this give max and min timing ... why run cworst, cbest, what does this tell us?

There is no clear relation between RC Corners and Maximum delay.
Some nets are resistance-dominated and some are capacitance-dominated and some are mix.

Add to that the fact that increasing delay is bad for setup timing but good for hold timing, and vice-versa for decreasing delay.

So all you can say is that by picking different R and C combinations you will get a different delay picture for your circuit. Not necessarily better or worst, just different.

Foundries try to restrict the number of corners people need to verify, so they pick two corners that are typically best and worst. Just keep in mind it is a simplification of what really happens.

***rcworst or cworst ***
They are different kinds of spef ( standard parasitic extraction file) which are extracted by the tool like QRC for post routing analysis for setup and hold violations.
we back annotate these exacted files for timing closure.