A read through the earlier posts might give you the clue ...
(a) the limitation is whatever rated thrust you are operating ... it is on this rating that the certification is based.
(b) if you are operating at a derate thrust limit, that lesser thrust becomes the limit for the takeoff .. ie one ought not to increase thrust beyond the derate value
(c) if operating flex, the limit is the rating on which the flex is based, whether max rated or derated ...