.if marks the beginning of a section of code which is only considered part of the source program being assembled if the argument (which must be an absolute expression) is non-zero. The end of the conditional section of code must be marked by .endif (see .endif: Endif.); optionally, you may include code for the alternative condition, flagged by .else (see .else: Else.).
The following variants of .if are also supported:
Assembles the following section of code if the specified symbol has been defined.
Assembles the following section of code if the specified symbol has not been defined. Both spelling variants are equivalent.