What is derating? why it is necessary?

What is derating?, why it is necessary, it is same for all means for drives, motors,and cables. ?

The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalogue) this is called derating.

The factor by which the ratings of component parts are reduced to provide additional safety margins in critical applications or when the parts are subjected to extreme environmental conditions for which their normal ratings do not apply.