Funny you mention chokes, because manufacturers vary widely on what CYLINDER bore is for a 12 bore (or any other bore for that matter). Cylinder bore is what all the other chokes will be based on, usually getting smaller in diameter by .005" with each step. Here are two examples.
Browning Invector Plus
CYLINDER .740 (5 notches)
SKEET .735
IMPROVED CYLINDER .730 (4 notches)
LIGHT MODIFIED .725
MODIFIED .720 (3 notches)
IMPROVED MODIFIED .715 (2 notches)
FULL .710 (1 notch)
EXTRA FULL .700
TURKEY .650 - .680
Beretta Optima Plus/Benelli Crio Plus
CYLINDER .725 (5 notches)
SKEET .720
IMPROVED CYLINDER .715 (4 notches)
LIGHT MODIFIED .710
MODIFIED .705 (3 notches)
IMPROVED MODIFIED .700 (2 notches)
FULL .695 (1 notch)
EXTRA FULL .685
TURKEY .640 - .675
Constriction is how far below cylinder bore the choke you are using is, and is measured in thousandth's of an inch. This is normally expressed as choke nomenclature...aka...MOD, IM, SKEET...etc. But one manufactures MOD can be another manufactures SKEET. This is why I believe that it would be more accurate to discuss what
diameter choke is being used rather than the constriction or choke nomenclature. That's never gonna happen, but it would be nice.
.015" difference between CYL bores (and all the others too) is a huge difference. Patterning your shotgun with the load & choke combination you intend to use is important. Pattern density at distance, how high or low does it hit with a specific sight picture...etc. There is a lot to learn at the pattern board. This also explains why my Benelli SBE3 patterns so much tighter than my friends Browning Maxus when using the "same" chokes.
I won't get into what they do in the UK because it only gets more confusing when discussing chokes expressed as a fraction.

I'll leave that to
@WAB.