Xilinx device routing architecture comprises interconnect resources of various lengths in each direction: North, South, East, and West. A congested area is reported as the smallest square that covers adjacent interconnect tiles (INT_XnYm) or CLB tiles (CLE_M_XnYm) where interconnect resource utilization in a specific direction is close to or over 100%. The congestion level is the positive integer which corresponds to the side length of the square. The following figure shows the relative size of congestion areas on a Xilinx device versus clock regions.
Figure 1. Congestion Levels and Areas in the Device View