ศาลาชุมนุม

I'm interested in how the T/E key works. I did a fair bit of googling but couldn't find much of anything.

Here's my guess:
When in Thai mode, pressing it switches to English mode and pressing it with Shift inserts a % character.
When in English mode, pressing it switches to Thai mode and pressing it with Shift inserts a ~ character.

There's also a Thai-only layout where the key is used to insert _ characters instead of swapping layouts; is that commonly used in Thailand? It's the behaviour I get when choosing Thai language in my English version of Windows 7.

The attached photo has ` on the English side, can the key be used to insert that instead of swapping to Thai? That's what it does in normal English (US) layout.

Edit: To clarify, I don't have a keyboard like this myself and I'm not trying to configure my own computer to work like this. I'm just curious to know how it works in Thailand.

There's separate logic to intercept the scan code for the physical key and the note the corresponding modifier settings. I'm pretty sure that if you set up the grave key to switch from English to Thai, you have to do something complicated (and possibly not well supported) to enter a backquote. Beyond that, I'm pretty sure it becomes system-dependent - fun and games can ensue if your switch 'character' changes position when you change keyboard.

That makes sense. I assume the backtick in the photo is just careless design. I'll do a bunch of experiments as soon as I get my hands on a Thai Windows.

You need a backquote for some programming purposes, so it's not an error.

You can get pretty close to Thai Windows from Windows XP if you make it the default encoding for your PC. Apparently that has been necessary to make backquote the language switch character. I suspect later versions of Windows will make it more readily available. However, backquote seems to be useful for it to be made easy to you like this, which is why obscure combinations of control characters are easier to set up.