VMS Help MACRO, VAX MACRO Assembler, Operand Formats, Relative deferred  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The address specified is  the  address  of  the  operand  address
    (a pointer  to the operand).  The assembler stores the address
    specified as a displacement from the PC.  Relative deferred mode
    can  be  used with index mode.

    @address

    address

    An expression specifying an address; the expression can  be
    preceded by  one  of  the  following  displacement  length
    specifiers,  which indicate the number of bytes needed to store the
    displacement.

          B^    Displacement requires one byte.
          W^    Displacement requires one word (two bytes).
          L^    Displacement requires one longword (four bytes).

    If no displacement length specifier precedes the address
    expression, and  the  value of the expression is known, the
    assembler chooses the smallest number of bytes (one, two, or  four)
    needed  to  store  the displacement.    If   no   length
    specifier  precedes  the  address expression, and the value of the
    expression is unknown, the assembler uses  the  default
    displacement length.  If the address expression is either defined
    later in the program or  defined  in  another  program section, the
    assembler considers the value unknown.