im not sure I understand what your asking. How is this different that putting a number on each class? you mean class and build specific sign ups?
Generalizing signups. A Warrior can spec for DPS versus tanking and still make a vital addition to a raid. A Priest can be Shadow or Holy. If a Druid signs up they can be Balance, Restoration, or Feral. Paladins can be Protection, Holy, or Retribution. So by breaking signups into roles which has been on the table for a while now it make is easier to run signups and know deal with approvals. Lets take Kharazan as the main example. Lets say your guild runs it with 2 tanks, 3 healers, and the rest DPS. Breaking the signups out as below will make it easier to lay that data out and let people sign up and know who can do what.
Healer
Tank
DPS