There are 3 level variations.
These allow for an increase in in the planned FDP of 30,60, and 75 minutes respectively.
The requirements in order to comply with these variations are quite lengthy and restrictive. 3 level 2 flights may be undertaken in a 28 day roster period.
2 level 3 flights may be undertaken in a 28 day roster period (3 for cabin crew).
Any level variation must form part of the companies approved FTL scheme. The compensating factors applied to these schemes (days off, reduction of overall duty hours etc) are designed to prevent the onset of fatigue, and are only allowed within very carefully controlled circumstances (ie specific flights)