CL2 is the designation for Class 2 Circuit wire and CL3 is the designation for Class 3 Circuit wire. Both are regulated under NEC Article 640, 725 & 820. They are both used for Video, Audio Interconnects, remote-control, signaling, and power-limited circuits. In plain English that means that CL2 and CL3 are rating types for most types of low voltage cabling such as: speaker wire, Cat5/6, and HDMI cables.
The difference between the two is that CL2 is rated for up to 150 volts while CL3 is rated for up to 300 volts. (NEC 725.71 F) According to NEC 725.2, CL2 offers some protection from electric shock while CL3 offers a bit more because it is rated to carry more voltage. CL3 may be used in place of CL2, but not the other way around.
This designation is very important when wiring homes. The rating relates to fire safety. When wiring homes during construction and in retro-fit applications, the wire is typically ran within the walls. This allows for a “Clean/Neat” look after the any A/V equipment is installed. It is important to ensure ALL wiring has a CL2/CL3 rating on the outer jacket. During construction, inspectors will check for this rating on the wire that is being used.