Normal overcurrent protective devices (fuses and circuit breakers) have two continuous current ratings; open air or in an enclosure. Group mounted is not a normal part of the individual device evaluation.
The result (and probably what you are referring to) is that devices are tested at 100% current in open air. But, as soon as they are put into any enclosure their continuous rating is effectively only 80%. If an enclosure has adequate ventilation and is tested with a specific device the assembly may have a 100% rating.
You will notice that this "derating to 80%" matches exactly with the NEC requirement of sizing protective devices and conductors at 125% of continuous loads.
