De-rated take off and flex take off ,some airlines do them both,for derated take off ,how do aircraft/engine manufacturer have these set reduced power settings meet certain regulations? since it might gets complicated when it comes to Vmcg issue.
Vmcg is taken into consideration in derated/flex TO calculations.
With a fixed derate, you cannot use full thrust on the remaining engines if you lose one. With reduced thrust (assumed temperature), you can use full thrust. I don't know what Airbus uses for flex.