Where is the macro verbiage used for button labels and tooltips defined?

[NegativeZero]

I have been playing around in the Button Editor and some of the predefined buttons have marcos/ aliases like the following:

Label   -> D:\ (#{b,21})
Tooltip -> #{b,21} D:\

I could not seem to find anything in the documentation as to how those work. Perhaps they are internal use only? When I tried changing the numerical values to experiment then it broke the association so the button went from saying "D:\ (New tab)" to literally "D:\ (#{b,21})"

[Mathias (Author)]

That are what text id from the language file they should use, so that the standard button get the correct language.
When doing your own button you can skip that.