First we need to separate two completely different things;
The Thai Embassies & Consulates scattered hither and yon across the globe are under the jurisdiction of the Ministry of Foreign Affairs. Each embassy/consulate has a great deal of latitude in deciding criteria for issuing visas. Think of them as whores from different go-go bars..
Case in point, some thai consulates will issue a year-long, multi-entry Non-Immigrant Type-B (business) visa to you if you write a letter yourself stating you want to go go thailand to look at business opportunities! Others want the normal "pile 'o paperwork" from the sponsoring company, (tax returns, list 'o thai employees, shareholders, etc). There's almost no rhyme or reason in the difference in criteria from one embassy to another.
FWIW: NEVER EVER believe what a consulate or embassy tells you about how things are done concerning visas/extensions inside thailand because they don't know their arse from their elbow about it. They issue visas to people outside the country, period..
Now, INSIDE the country visa & extensions are controlled by Thai Immigration which is a division of the Royal Thai Police. They too have great discretion as far as how high they want us foreigners to jump when securing extensions of stay. In perusing the thai language version of Immigration rules (which BTW: is the version they follow, NOT the b/s engrish translation provided by law firms here), almost every clause has "or other documentation as requested by the issuing office" <- that's their "out". They can and often do ask for totally bullshit documentation.
Another case in point. I was at Chaengwattana and the guy ahead of me in the queue in section "L" (over 50, marriage, raising kids) was getting his 11th yearly extension of stay based on being over 50. Now, he was kind of a twat to the officer telling her oh-so loud that he'd been here 10+ years, knew what was what, etc. Even as confrontational as I can be with those officers, I've found bloviating like that BEFORE you get your extension can yield less than optimal results. She asked him for a copy of his lease, of course, which he didn't have, as it's not a "real requirement". Actually, she'd already stamped the yearly extension into his passport, but she used a ruler and crossed it out. Then she made him go home, get a copy of it, come back, re-queue up and gave him his extension.. She did this simply because he was a little rough with her.
I've found treating immigration officers like spoiled little children works better than treating them like adults..
Anyway, that's all I got guys.