boat capacity tag with respect to weight is calculated by testing. basically the boat is floated, and weight is added until a percentage of free-board is left.
regarding weight of the motor, outboard capacity plates are without motor. the motor weight is part of what the boat carries. so if your capacity plate says 50hp and 500#. no more than 50hp and no more than 500# of motor, gear, people etc.
My boats tag says 960 lbs and max 90 hp. I have to re-power my boat and was looking at a a 60hp( weighing 242 lbs) a 90 hp( 303 lbs) a 115(347 lbs) so does that mean if I put the 60hp on the boat I can carry 61 lbs more weight than if I put the 90hp on and if I put the 115hp I would have to carry 44 lbs less weight ?