CMaier seems to be confused a little so I'll comment to maybe clear some of that.
Light Varmint is 22 cal or better, 10.5lb limit
Sporter is 6mm or better, 10.5lb limit
Heavy Varmint is 22 cal or better, 13.5lb limit
Unlimited is unlimited...shoot any safely operated rifle
You could shoot a 22 cal in light, a 6mm in sporter, a heavier 22 in heavy, and an all out unlimited in the unlimited class. Conversely, the sporter rifle meets the rules for all four classes. If you had a heavy varmint rifle that shot better than your sporter you would likely shoot it in heavy varmint and perhaps unlimited...but you can't shoot it in all 4 classes.
Soooo..many folks that don't have the time or money to have more than one rifle simply work toward a good shooting sporter rifle. Actually, that's the better way to go in my opinion....except for the 10 shot unlimited class. You really need a quick shooting rail for that one....but keep in mind you can shoot a sporter class rifle nonetheless.