GCWR covers the tractor, the trailer and the cargo together. It matters for CDL class, because a combination with a GCWR of 26,001 pounds or more, where the towed unit is over 10,000 pounds GVWR, needs a Class A license.
GCWR is a rating and not a legal axle or bridge limit. The legal weight on the road is still set by gross, axle and bridge formula limits.
Regulation: 49 CFR 383.91.