Drawing a ternary diagram:

ternary
define source sgte_sol !
define system 'Fe,Cr,Ni' !
set temp 973 !
class absent phase(*) !
class normal phase(fcc_a1,bcc_a2,sigma) !
compute diagram auto_ternary !

The section on binary diagrams explains how to label any area.

Remember a [Return] twice to leave the graphical display, or a [Return], x [Return] to leave and close the current output file.