In this particular case not a PCB but microprocessors.
Global routing is a high-level routing approach: routing millions of nets (= connections usually between 1 output pin of a gate and N input pins of other gates) on a grid graph with potentially billions of nodes is an extremely difficult Steiner tree problem.
To get good solutions, a common approach is to first solve the problem on a much coarser graph where edges have capacities. I.e. instead of having to place all connections disjointly, edges in the graph can accommodate a number of parallel connections.
A later detailed routing step will produce the actual disjoint routes while following the outline that was obtained during global routing.
Global routing is a high-level routing approach: routing millions of nets (= connections usually between 1 output pin of a gate and N input pins of other gates) on a grid graph with potentially billions of nodes is an extremely difficult Steiner tree problem.
To get good solutions, a common approach is to first solve the problem on a much coarser graph where edges have capacities. I.e. instead of having to place all connections disjointly, edges in the graph can accommodate a number of parallel connections.
A later detailed routing step will produce the actual disjoint routes while following the outline that was obtained during global routing.