To add one bit to what Fred said the Brits do use the I/C choke. It normally ran about .006". I believe most continental makers do skip from ¼ choke to Cyl as he said. Most US I/C chokes are in reality British or Continental ¼ choke, having often .010" or more constriction. I am talking here of older guns, I do not know what constriction current guns have.
Miller
PS; the Stars are *=F(1/1); **=3/4; ***=1/2; ****=1/4 & CL=Cylinder on Italian guns.
Browning uses * = full; *- = I/M; ** =Mod; **- = I/c (really ¼) **S = skeet; & *** = cylinder.
Some choke tubes are marked with a fraction over 10, as 3/10. This would equal .3MM or about .012".
Miller
