I retired as a teacher in Thailand in 2010. When I last renewed my work visa in 2009, there was no requirement to get this. What is this letter ? What are they certifying ?

Not sure - but one school I was at never bothered to report any foreign teachers.

They claimed they all were hired help, paid as casual laborers or something. No one got a B-Visa

It's a permission from the MoE to apply for a non-immigrant ED or B (teaching) visa at a Thai embassy or consulate. Not all embassies and consulates require one because I think the staff at those consulates or embassies don't actually know what it is. Your school needs to prepare the paperwork and then submit. In Chiang Mai, it can take 2 weeks to complete.

It is one of the things that slows down the process of getting the paperwork in order.

It's basically just a letter from educational area xxx, where they state that they're informed that you're working at his particular school.

Works pretty fast at smaller places in lower northeast, you can get the document within two days. ( If the boss is on duty)

I asked the question in relation to the case of a friend. Immigration insisted that he get this letter, before RENEWING his visa He did not submit any information to the Regional Office that he had not already submitted to the Immigration and Labour Dept's. They told him to come back the next day to get the letter, which once he got it, looked to contain nothing more than the info. from the ID page of his passport, and their approval for him to work in the area.

I asked the question in relation to the case of a friend. Immigration insisted that he get this letter, before RENEWING his visa He did not submit any information to the Regional Office that he had not already submitted to the Immigration and Labour Dept's. They told him to come back the next day to get the letter, which once he got it, looked to contain nothing more than the info. from the ID page of his passport, and their approval for him to work in the area.

Which was basically already explained in the post before.