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...

Examples:

• 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... :-)

Item

Identification
	Sequence	60
	Name	P1168_REQUEST
	Display As	Display Only
User Interface
	Region	Value of Request
Source
	Source Used	Always, replacing any existing value in session state
	Source Type	Expression
	Source value or expression	'REQUEST = ' || :REQUEST

Button

Name
	Button Name	TEST_1
	Text Label / Alt	Test 1
Displayed
	Sequence	20
	Display in Region	Value of Request
Behavior
	Action	Submit Page
	Execute Validations	Yes

Button

Name
	Button Name	TEST_2
	Text Label / Alt	Test 2
Displayed
	Sequence	50
	Display in Region	Value of Request
Behavior
	Action	Submit Page
	Execute Validations	Yes

Branch

Identification
	Name	TEST_2_BRANCH
Execution
	Sequence	10
	Point	AFTER_PROCESSING
Behavior
	Type	Branch to Page or URL
	Target	f?p=&APP_ID.:&APP_PAGE_ID.:&SESSION.:TEST_2:&DEBUG.:::
Server Side Condition
	When Button Pressed	TEST_2