I've seen wheels listed with a specific wheel code in various ads and one of them got my interest.
The "Competition Wheel" is always a 15x6. After the 15x6 stamp, which is usually written as " 1 5 x 6", is usually the code "JK", written as " J K".
This code, it turns out, is an SAE designation for the bead profile. Bead profile is the width, height and radius of the outer portion of the rim (see pic).
The bead profile is listed in SAE papers and can be J, JJ, JK as well as many other codes. "J" is today's most popular bead profile.