RC corner
Beginner
One extreme of how thick or thin the factory makes the wires.
Novice
An extreme of how the factory might make the wires: for example the version with the most capacitance (wires printed wide and close together) or the one with the most resistance times capacitance (thin, narrow wires). Extraction is run once for each.
Expert
Common corners are Cworst/Cbest (maximum/minimum capacitance) and RCworst/RCbest (maximum/minimum resistance × capacitance). Short nets, whose delay is mostly the driver charging capacitance, are worst at Cworst; long nets, where wire resistance matters, can be worst at RCworst. RC corners are paired with device corners on purpose rather than run as a full cross product.