The class of vehicle licence is determined by the Gross Vehicle Mass (GVM) found on the registration sticker or the compliance plate of the vehicle. There are no separate licence classes for trucks and buses, it is determined by the weight of the vehicle & number of axles (see below), and a licence from a higher class lets you drive a smaller one.  Typically an LR, MR or HR can tow a trailer as long it is less than 9 tonnes and within Gross Combination Mass (GCM) limits.

Click here to view the official licence class information from the South Australian Government.

Allan Miller's "Plain English" Licence Class Guide

What you can drive?
	Light Rigid - LR: small delivery van or bus up to 8 tonnes. .
	Medium Rigid - MR: a larger rigid truck or bus, with maximum two axles. Can also drive a special purpose vehicle up to 15 tonnes (e.g. a bob-tail prime mover or tractor).
	Heavy Rigid - HR:  rigid vehicle with three axles, also including an articulated bus.
	Heavy Combination - HC: semi-trailer, or a rigid vehicle towing a trailer more than 9 tonnes.
	Multi Combination - MC: B-double or road train.