Rep schemes are always determined by the type of exercise and the type of goals found in each program. If the gains are for complete size/hypertrophy, there can be a few ways to approach training. In my experience, I have almost always found the most success from doing one major strength movement actually focused on strength gains. For example, on leg day an athlete will come in and do 4 sets of 3-5 on back squat to focus on strength and to stimulate the central nervous system. With hypertrophy as the goal, there will be drop sets performed with the back squat after the strength sets. This may look like two sets of 10 reps, unbroken (no pausing at any position of the rep). This will hold a large amount of time under tension (TuT) and lead to impressive trophism.
Following the main strength lift (back squat), I would begin to incorporate very high rep sets. Many strength coaches believe the optimal rep range for hypertrophy is between 8-15 reps, this can be true to an extent BUT as the athlete becomes more advanced, these sets MUST be extended to 20+ reps!!! Strength athletes love to shy away from these sets because they can be quite brutal and mentally challenging but this is where the athlete should certainly trudge forward and work in the high rep/long duration set because it can improve their motivation and lead to incredible gains in the weight room.
Keep in mind, one of the greatest bodybuilders ever Dorian Yates, consistently destroyed his legs with 50 rep squat sets to achieve massive hypertrophy in his quads, hamstrings and glutes. Yates is one of the mentally strongest athletes to have walked our planet and is now a practicing yogi! After mastering these sets, spiritual gains may even be found!!!