As far as I understand it, a control constant is named in this way:
A prefix that describes the control type plus it's purpose
An underscore charater
A name / description of what it does.
Here is an example:
BS_LEFT
The "B" means it's a Button control.
The "S" means it's a style, and usually used when creating the control.
On the other side of the underscore we have "LEFT"
This means the text in the button will be left justified.
So you could read BS_LEFT as "Button control, Style, Left justify text" (or
something like that).
Common letters before the underscore are as follows:
M_ means it will send a message
N_ means it will send a notification
S_ means it's a style.
But there are others I won't go into for now.