Special variables in PL/I are termed "Pseudo-variables".
They are used only on the LHS of an assignment statement.

They include:
   REAL to assign the real part of a COMPLEX variable;
   IMAG to assign the imaginary part of a COMPLEX variable;
   SUBSTR is used to assign part of a string (used on the LHS of an assignment);
   STRING when used on the left-hand of an assignment;
   UNSPEC used to assign a bit pattern to a variable;
   ENTRYADDR is used to assign an address to an ENTRY variable that is
             to be invoked;
   ONCHAR resets the current value of the CHAR built-in function (when
          a data conversion error occurs, and it is desired to re-attempt
          the conversion);
   ONSOURCE assigns a new value to the ONSOURCE built-in function
            (may be used when a data conversion error occurs,
             and a re-try of the conversion is to be attempted with modified data);
   ONGSOURCE assigns a new value to the ONGSOURCE built-in function
            (may be used when a data conversion error occurs,
             and may be used when a re-try of the conversion is to be
             attempted with modified data).