VMS Help DTM, MODIFY, VARIABLE, Command Qualifiers, /SYMBOL  *Conan The Librarian (sorry for the slow response - running on an old VAX)

   Changes a variable from a OpenVMS logical name to a OpenVMS symbol. You
   must define variables used as symbols as either numeric or string
   variable types. You cannot specify both the /LOGICAL and /SYMBOL
   qualifiers with the same MODIFY VARIABLE command.