I think the answer is "whichever set of rules the club is using".

Some have IDPA divisions, and some have just "Limited".

At the nationals, you can either run in IDPA or Limited, but not both. IDPA shooters compete as a subcategory of open (I'm guessing to separate them from the Limited shooters).

The "steel master" award is the compilation of open, limited, and .22 scores. The IDPA category appears to be a concession for folks who don't want to upgrade to Limited standards for the match, and don't want to feel "disadvantaged".

I personally will be shooting Limited with my production rig, which is SSP legal apart from the outer belt not going through the pant loops.

A58581 / A28264