A High Power round usually describes ammo that is the same caliber with a more or different powder that creates more pressure. The chambering has to be reinforced for the extra pressure.
A real life example would be the .357 magnum round. It is the same caliber as a .38 special, but uses a slower burning powder to create more pressure.
Usually, the higher power ammunition will have different length casing to differentiate between it and a standard round of that caliber (again look at the .38 special and .357 magnum). A lot of the times this is just for safety though, so the two types don't get mixed up.
That different case length is what makes automatic weapons unable to use both rounds interchangeably. One of my house rules is that HPC revolvers and break action weapons can use normal and HPC rounds.
Then again, I also think that High Power ammo should just modify standard ammo types instead of being a unique type. After all, if you look at high power rounds in real life, they come in just as many varieties as standard rounds.