One of the most difficult specs to understand is brake rotor thicknesses. Below are the best descriptions I have ever read:

Nominal Thickness:
Nominal thickness is the thickness of the rotor when it is new. This thickness is not usually used on a daily basis, but is listed in most specification books.

‘Machine To’ Thickness:
This is the thinnest a rotor can be machined to and still be put back into service. The purpose of having a machine to thickness is to provide enough rotor material to last the life of one set of pads. The assumption is that if the pads are replaced and the rotors are over “machine to” they should not experience enough wear to allow them to go below discard thickness through the life of that set of pads. The average difference between nominal and machine to thickness is .050″ to .060″. The typical difference between machine to and discard is .015″.

Discard Thickness
The discard thickness of a rotor is the thickness at which the rotor should be replaced. The common understanding of the definition of discard thickness is that it is a heat-related dimension. It is generally understood that if a rotor is at or below discard thickness, it cannot dissipate the heat generated. This is not correct. Discard thickness is the thinnest a rotor can wear to so that in the event the brake pads wear to nothing, the caliper piston won’t fall out of the caliper housing. It has nothing to do with heat.