From what I know, the discard thickness means that in someone is going to machine the rotor, they cannot discard more than 0.89mm during the process. Minimum thickness should be somehwere like 20+mm and is different for front and back rotors. I am not sure if Jeep follows similar standard or not.

 

No, it means that if the rotor is thinner than 0.89" (inches, not mm), it must be discarded.