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})"