I had to search the sources to realise the fat filesystem type is called MSDOS. Maybe at least a header can be mentioned in disklabel(8)?
Index: disklabel.8 =================================================================== RCS file: /cvs/src/sbin/disklabel/disklabel.8,v retrieving revision 1.107 diff -u -p -r1.107 disklabel.8 --- disklabel.8 2 Jan 2012 04:38:00 -0000 1.107 +++ disklabel.8 6 Apr 2013 13:57:19 -0000 @@ -300,6 +300,8 @@ prompt will exit the editor. At prompts that request a size, .Ql * may be entered to indicate the rest of the available space. +For a list of available filesystem types see the fstypenames table in +.Pa /usr/include/sys/disklabel.h . The editor commands are as follows: .Bl -tag -width "p [unit] " .It Cm \&? \*(Ba h -- Michal Mazurek