I have been required to install these many times, it only needs to shut down the boilers. I have also had engineering specs that required closure of the main gas valve. I would still peruse the aforementioned NFPA articles just to be familiar with the specific verbiage.
Just curious, was that a second redundant valve or something of that nature?
I have seen two valves in series but both operate from the main process control, this just in case one valve fails to close the other hopefully does, and this happens on every control cycle not just when an E stop would be involved. Bad thing I noticed was there was no way of indicating either valve failed to close, so operator would never notice a problem until both failed to close. This is on 30 year old burner for a drying system, possibly would be done different today.