Hm, I think I got it:
IF a variable FILEMGR is defined in environment, then mnttool will mount the slected partition AND open that file-manager (fluff, xfe, whatever) with that partition already selected AND it close itself. Nice.
However, if no FILEMGR is defined (is empty string) then mnttool will mount the partition AND stay alive for next command (mount another partition). Very clever, but nowhere explained in the wiki neither in forum, book etc.
PS: "strings which (mnttool)" found another variable, MNTTOOL. I wonder what it will do undocumented.
From source code it seams that it defines position (x y) to show on desktop, maybe from top-left (found by try and error).
PS2: in aterm, the message "XOpenIM() failed" remains.