Class I - Typically installed on very small compact vehicles. Size 1.25in X 1.25 in 150 LBS Tongue Weight Rating (TWR) on most Class I hitches.
Class II - Typically installed on Mid-Sized vehicles. 1.25in X 1.25in 350 LBS TWR on most Class II Hitches. (HA Class II can be used on this hitch for small carriers and light weight applications, please check with you vehicles owners manual for max load capabilities of your vehicle.
Class III - Typically installed on standard size vehicles, SUVs, and trucks. Size 2 in X 2in with an average TWR 600 (But weights can vary by Hitch MFG and Vehicle setup)