We get asked this question all the time !
A zero-clearance box allows you to put a wood or gas heater into any home. This zero-clearance box above is for a wood heater, it has two layers which allows the hot air flow to pass through and around the chambers allowing the temperature to cool down to a safe level, so it can be installed into a wood frame.
In gas zero clearance boxes, heaters and the zero-clearance box are generally built all in one. Even thought they are call zero clearance, all heaters still have their own specified clearances that need to be followed to comply to regulations. You will find these in your owner manual.