What additional documents are required for a Thai business visa application?

Sounds like you're applying for a Non-B visa for "doing business" in thailand. Years ago those visas could be had for the asking at most of the honorary consulates in the US, by just saying you're thinking of "doing business in thailand".

They tightened up on them considerably and I haven't heard of anyone getting more than a 90 day single entry before the covid sh*t show closed the country down 😮

Since it's re-opened I haven't heard of any Non-B's issued just for 'doing business' (the ones they issue without you holding a valid work permit or proof you applied for one)

Now that's not saying that they don't issue them, only that they are few and far between.

I let your post on to see if anyone on the group has gotten one. You might have to resort to calling the consulate you're applying for the eVisa thru and seeing if you can get your answer that way.