Never overlook safety when making a cabling decision. The wrong cable in the wrong place can lead to significant safety hazards, as different coatings and materials used in the cable design dictate that solutions are used in specific ways. For HDMI wires, most solutions are not designed for in-wall use. Instead, a standard HDMI cable should only be used externally to ensure safety. However, any wire with the CL2 or CL3 designation has been treated in such a way that it can be used within the wall.
CL2 and CL3 HDMI cables are coated with a special material that releases less toxic gas when the cable catches on fire. The lower toxicity makes the wire safe to burn, making it a much better alternative to standard HDMI cables in the event of a fire or overheating.
This added safety is key because it provides the protection necessary to put HDMI cables inside walls while still complying with the National Electric Code. CL2 cables are certified to carry up to 150 volts and are designed for residential deployments, while CL3 wires can handle 300 volts and are intended for use in commercial settings.