I would think that the 'baseline' is the variant with the highest thrust rating, i.e. the RB211-Trent 980-84,
sealevel static TO thrust rating 84,098 lbs, flat-rated to ISA+15C. The thrust levels of that rating must be achieved without exceeding operating limits of RPM and TGT. The manufacturer selects rating(s) so that they they have some margin relative to the operating limits. The lesser ratings are 'derates' and have greater margins than the highest rating. In
TO ratings Trent 900 the various ratings are shown at the flat-rated temperature. The dashed line shows schematically the thrust corresponding to the operating limits, whichever is most limiting. The line is drawn arbitrarily, because the documents don't tell us where it lies.
It's actual location will be different from engine to engine, and the margin will reduce as the engine deteriorates during its service life.
Prior to certification, proof of integrity of the engine operating at the declared operating limits must be provided by analysis, component testing and, finally, by a 150 hr endurance test which represents an accelerated life cycle of the engine. 'Modelling' is a design tool, not proof of compliance with airworthiness standards.