If anyone before has been IRC during rants on the usage of exit vs menu, I am simply making this topic to report the facts gathered from casio's UI etiquette:

• MENU is reserved only to return the the operating system's main menu. Few exceptions exist, such as optimizing the storage memory or running the first-run setup.
  
• EXIT backs out of a window/operation, such as cancels editing a line, closing a MsgBox, ...
  
• QUIT is used to return to an add-ins default view, except when inside of MsgBox's or system dialogs (which will instead act as EXIT).
  
• AC/ON is used to break from certain conditions and operations, such as program execution, clipping, and the catalog.

The above can be seen from various places in the OS. Some odd exceptions apply. Remember, this topic is not intended to have a debate about this.

<edit>

An addition must be made here. Text input on the Prizm defaults to insert mode. In that mode, DEL key always is a backspace key.

°∠°

Please do not forget, that the most of Prizm developers create their addins for fun, and (that is more important) for free. They spend their time to create useful (or less useful) software for you.
It is their freedom to assign function keys to any functionality they want.

[Menu] button is not reserved for anything.
For example I plan to update FXTerm and I add menu for setting different things. It will be activated by [Menu] button.
The menu will contain "exit" item to quit the FXTerm, but user will have to select it.

There are many different scenarios, where [Menu] button may have eminently different functionality than "return the the operating system's main menu".

Why not use [shift]+[menu] for setup?

You are, of course, welcome to do whatever you want with the keys. AHelper isn't telling you what to do, simply stating what he's observed from Casio's products.
That's really not relevant to this discussion, however true it may be.

Use the "EXIT" key for exiting instead of MENU and I've just found one reason not to you use your add-in - unfortunately for me, since there are many good add-ins out there I don't use because I can't withstand this lack of coherency (I find it very annoying I have to try one or two keys every time I want to get out of something, before I get the correct one), the same way I don't like when e.g. on Windows some application has the close/minimize buttons on the left side instead of on the right side.

Because...
- [Menu] is faster
- you do not quit the application by accident
- no need to handle multiple buttons simultaneously
- align with the whole architecture of the addin
- any other reason

As long as you make it open source so I can fit the keys to my tastes...

Yes, it's true, I overlooked that, because there is a link to this thread (in http://www.cemetech.net/forum/viewtopic.php?t=7950&start=20) in context which makes me feel like "this must be done this way".
Theme like "Prizm Icon Design Guidelines" makes me sick.

They're just guidelines, no need to overreact. There's something to be said about conforming to the UI standard of the platform--I prefer my icons to look, at least a bit, like the Casio ones, and I'd rather stick to the controls they seem to prefer. That's all choice, though. Very few people will fault you for going outside of that (for example, Kerm chose to use shift for Tetrizm as though it were 2nd on the TIs, which works. I chose to use the F keys in minesweeper, which also works. No one has, to my knowledge, complained too loudly about either).