This is something that Mike Tyson's trainers used to do when he was starting out. They would give each different punch a number, and shout out the numbers while he was sparring or doing bagwork. He would then immediately throw that combo.

This is one of the things that made Tyson such a devastating combination puncher, as they would all flow out with speed and power.

I would think that, now it has become popular, different trainers will have different numbers for the punches (just to confuse things)