So... I'm studying for my engineering license and I learn something new every day. The 2018 International Fuel Gas Code Section 626 covers "gas-fired toilets."
I strongly suspect that's a reference to methane, aka "natural gas" although in an older, historical sense, it would be hydrogen, aka "illumination gas", which would make sense under the current hydrogen revival.
Gas-fired toilets shall be tested in accordance with ANSI Z21.61 and installed in accordance with the manufacturer's installation instructions.
626.2 Clearance
A gas-fired toilet shall be installed in accordance with its listing and the manufacturer's instructions, provided that the clearance shall in any case be sufficient to afford ready access for use, cleanout and necessary servicing.
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