VMS Help MACRO, VAX MACRO Assembler, Directives, .IF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    A conditional assembly block is a series of source statements that
    is assembled  only  if  a  certain  condition  is  met.   .IF
    starts the conditional block and .ENDC ends the conditional block.

       .IF condition argument(s)
            ...
            range
            ...
       .ENDC

    condition

    A specified condition that must be met if the block is to be
    included in  the  assembly.  The condition must be separated from
    the argument by a comma, space, or tab.

    argument(s)

    One or more  symbolic  arguments  or  expressions  of  the
    specified conditional  test.   If  the  argument  is  an
    expression, it cannot contain any undefined symbols and must be an
    absolute expression.

    range

    The block of source  code  that  is  conditionally  included  in
    the assembly.

    Each .IF must have a corresponding .ENDC.  The .IF directive
    contains a  condition  test  and  one  or  two  arguments.  The
    condition test specified is applied to the argument(s).  If the
    test is met, all VAX MACRO statements between .IF and .ENDC are
    assembled.  If the test is not met, the statements are not
    assembled.  An exception to this rule occurs when you use
    subconditional directives.

    VAX MACRO permits a nesting depth of 31 conditional assembly
    levels.