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Global routing is basically an estimation of routes required. Detail routing is the actual wire routing that happens and that can be manufactured. Global routing will talk in terms of Number of Routing Resources available and Number of routing resources required. It will split the entire floorplan into equally sized logical elements known as Buckets. Then, it will try to find out how many resources available on each metal level and how many required ( based on No. of pins within the bucket and within its close vicinity). Then calculate a ratio to identify whether there is overflow. If there is overflow for many buckets within a particular area, it determines that the particular area is congested.
Detail routing is actually the proper wire routing. The tool creates the wire and explicitely connects it with pins associated to each other. If the tool cannot route avoiding DRCs/ density of routes in pne particular area is very high , then the terms that the particular area as congested.