RUDY
Beginner
A quick way to guess where wires will pile up, by spreading each wire evenly over the box around the parts it connects.
Novice
Rectangular Uniform wire DensitY: a quick congestion estimate. Each connection’s expected wire is spread evenly over the rectangle around its pins, and the amounts from all connections are added up tile by tile to make a congestion map.
Expert
Per net, a density of over its bounding box, summed over nets. Router-independent and cheap, so placers call it inside the loop (OpenROAD gpl uses it for cell inflation). It ignores layer assignment and detours, so confirm with a global route.