The reason these are "undocumented" is because some of them will cause the tree to be drawn kinda funny. Additionally, they are not universal, won't work in many contexts, and may be ignored or overridden. But, in any case, here is a table of the possible values from the FLTK documentation:

@. Print rest of line, don't look for more '@' signs
@@ Print rest of line starting with '@'
@l Use a large (24 point) font
@m Use a medium large (18 point) font
@s Use a small (11 point) font
@b Use a bold font (adds FL_BOLD to font)
@i Use an italic font (adds FL_ITALIC to font)
@f or @t Use a fixed-pitch font (sets font to FL_COURIER)
@c Center the line horizontally
@r Right-justify the text
@B0, @B1, ... @B255 Fill the backgound with fl_color(n)
@C0, @C1, ... @C255 Use fl_color(n) to draw the text
@F0, @F1, ... Use fl_font(n) to draw the text
@S1, @S2, ... Use point size n to draw the text
@u or @_ Underline the text.
@- draw an engraved line through the middle.

I see that the document says "points", but that is unlikely. It may be points, but I suspect it is meant to read pixels.