In their discussion, Eric Moshier quotes Samantax Antone who wrote:
Another bonus is that a masonry heater is listed in the IRC so you can legally build them and your insurance company will cover them.
A smaller masonry heater for heating ~1,000s.f. normally only needs 200 firebrick.
As I have not gotten to reading the Rocket Mass Heater Builder's Guide in its entirety yet, I do not fully know the context that IRC is being used, but I think this is a reasonable guess as to what it means.