Derating Current

In what context?

for the cable

A cable's ability to dissipate heat is limited by its construction and the environment in which it finds itself. Derating is necessary to ensure the cable survives without damage while the circuit protective device operates. Otherwise there is a risk of fire.

BS7671 ("the Wiring Regulations") gives algorithms whereby any cable's derating may be assessed.

Derating prevents the powered circuit's ratings (the point of destructive stress on the most vulnerable element) from being approached closely or exceeded over the realistic expected range of operating conditions, both inherent and environmental. It is necessary...both technically and from a warranty/liability perspective.