GCell (global routing cell)
Beginner
One square of the coarse grid that the planner lays over the chip. The planner counts how many wires must pass through each square.
Novice
One tile of the coarse grid the global router lays over the chip, usually a dozen or more tracks on a side (OpenROAD’s default is 15 track spacings of layer M3). The router only tracks how many wires cross each border between tiles, on each layer, and compares that with how many tracks cross it.
Expert
Bigger GCells make global routing faster but hide local trouble such as cells with many pins close together and the edges of blockages. A border’s capacity is its track count minus blockages, pre-routed wires and any derate you set. Demand counts wires and, in some routers, vias and pins.