Here are the details spelling out the differences between 12 ga Standard steel shot loads vs High Performance:

Standard:
Diameter of shot no greater than 3.25 mm. Comparing to US shot sizes, US 4 shot would be too large at 3.3 mm. US 5 are 3.05 mm
Velocity measured at a point 2.5 meters from the muzzle cannot exceed approximately 1300 fps

High Performance: (Steel shot proof required)

Velocity at least 1400 fps

If shot diameter is greater than 4mm (British BB or larger), barrel must be less than half choke

Those are the CIP regulations.