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

    The register contains the address of the operand.   After
    evaluating the   operand  address  contained  in  the  register,
    the  processor increments the contents of the register by 1, 2, 4,
    8, or  16  for  a byte,   word,   longword,   quadword,   or
    octaword   respectively. Autoincrement mode can be used with index
    mode.

       (Rn)+
       (AP)+
       (FP)+
       (SP)+

    n

    A number in the range of 0 through 12.