Library /sys$common/syshlp/decset$envmgr.hlb cgui, macdef PopForm, macexp TxtF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Type a new (or edit an existing) macro expansion string in this
    field.  The expansion string associated with the selected macro
    definition is displayed in this field.  Use this field in
    conjunction with the Enter and Replace buttons to add or change
    macro definitions in the Macro Definitions list.