Class 2 and Class 3 definitions are covered in the NEC (National Electrical Code) Article 725 "Class 1, Class 2 and Class 3 Remote-Control, Signaling, and Power-Limited Circuits".
Class 2 Circuit is "Wiring System between the load side of a power source and the equipment. This circuit is power-limited, is considered safe from fire initiation, and provides protection from electric shock".
In Class 2 circuits, Class 2 cables are used.
Class 3 Circuit is "same as Class 2 circuit, with higher voltages permitted. ".
In Class 3 circuits, Class 3 cables are used.
"CM" is a listing requirement also from the NEC (Article 800-51) and stands for "Communications General Purpose". CM cables may not be used in plenum and riser spaces.
Hope it helps,
Roman Kitaev, RCDD