First numbers,3215 are scale (1/32 scale) and, rim size in the real world (15")
32 13 would be 1/32 scale and, 13" in the real world.
Next number is the width at the rim (the part where the tire sits) .200, .275, ,380 etc
Third number is the bore size (axle size) 062 is 1/16", 093 is 3/32" , 125 is 1/8", 540 is 5-40 threaded to fit the older threaded axles used in the sixties.
Last number is the insert diameter, so, you know what inserts will fit what wheel. First three numbers of my inserts are the outside diameter of that insert. BW450-01 is a .450" dia insert, BW375-01 is a .375" dia insert.
So, that part number, is a 1/32 scale 15" wheel that is .200 wide at the rim to fit a 1/16" axle, and, takes a .450 diameter insert.
Hope this makes sense.