Welding must be his area of expertise, I imagine 98% of certifiers wouldn't know enough about it to even think to ask such questions. Probably overcomplicating things a bit though, in all the research I've done when reinforcing my chassis I can't remember seeing any mention of steel grades or any special procedures. Most reinforcement plates out there don't even mention steel grades and ones that do are mild steel. I imagine they they would be made to match the rest of the chassis they're being welded to.
Might not be much help but here's a full manual for the CMP reinforcement plates that go over the prep and welding procedure. It doesn't have recommended weld settings but I asked for advice on that front directly from them at the time and got the following response. Someone with more welding knowledge might be able to deduce the steel grade from the voltages mentioned? I've no idea how that works. These guys have developed the kit, have installed it on numerous cars so do know what they're doing.
Doesn't answer the certifier's question directly but if you have no luck in your search for information, maybe he'd be satisfied enough for you to follow the tried and tested procedure laid out in the manual?