The Check Call window performs two functions: first, it will show you if the ham radio station has been worked before in the contest. Second, it will show if the ham radio station is needed as a multiplier on a different band. Shown here:
Using K4GMH as our example, the window shows that we already worked K4GMH on 20 meters as QSO 1 at 1807Z. See the ‘OK’ next to 80, 40, 15, and 10? That means that we have worked the United States on those bands and do not need K4GMH as a multiplier. If it said ‘New Country’ next to any band, it means that we do need the United States as a multiplier on that band.