Back in the day, American had D6Us and D6Ls. D6Us were unlimited. D6Ls were limited to the physical number of jumpseats installed on the plane, regardless of the load. The list was down to just a few carriers last I heard and I don't know if it ever fully went away or not.

The funny thing was, if the jumpseat was taken by a D6U and there was a D6L waiting, the D6L would be left behind.