According to the documentation (and online Help), when you select "Submit Page" as the Action for a button, it will "Submit the current page with a REQUEST value equal to the Button Name."
Well, that turns out to be not true - at least in recent Apex versions (I noticed it in both 20.2 and 22.2)
I could have sworn it worked fine in the past...
- The TEST_1 button does a simple Submit, which according to the documentation should set the value of REQUEST to TEST_1. Which it doesn't.
- The TEST_2 button does a "Submit Page", and has an associated Branch which explicitly sets the value of REQUEST to TEST_2. This works.
Note: Using "Redirect to Page in this Application" instead of "Submit Page", and explicitly setting the value of REQUEST, partially works. It does set the value for REQUEST, but other page items are not submitted of course. But that may be just what you need in some cases... :-)