Max power assurance runs are exactly that. It is used after an engine change for instance, IIRC there were two targets 50% and 75% the idea being to 'shoot' for a particular N1 based on temperature and field elevation, the resulting figures for thrust lever angle/ N2/ Fuel flow/ EGT had to be below the figures as set out in the manual. This then guaranteed that a max. power takeoff was achievable without exceeding any limits.