Derating of a Generator?
The term derating generally refers to over specifying a device or component in order to build in a certain level of tolerance or to increase the lifespan of the device.

Specifically, with a generator, you will have a finite amount of power it can produce.  When you approach that power limit the generator starts to labour more and more - like a car driving up a hill - and this puts extra strain on the generator and ultimately shortens its lifespan.

To derate it you would basically specify a more powerful generator than you need, so that when you are at your peak power consumption the generator isn't struggling.

There's no hard and fast rules about it, just what is easy and "available".

Say you have a peak power consumption of 9.8KW.  A 10KW generator would do, but only just.  Any unforeseen power consumption could have a critical effect, and during peak consumption the generator would be running pretty much at maximum.

Moving up to the next easily available generator, say a 12.5KW generator (whatever the shop has), would ease some of that burden and give you an extra margin for error.

Derating can also help to offset the unforeseen environmental changes, such as a heat wave, causing tolerances to become more critical, especially in heat producing equipment like a generator.