#elif lets you create a compound conditional directive. The #elif expression will be evaluated if neither the preceding #if nor any preceding (optional) #elif directive expressions evaluate to true. If a #elif expression evaluates to true, the compiler evaluates all the code between the #elif and the next directive.
#elif symbol [operator symbol]...
where:
== (equality)
!= (inequality)
&& (and)
|| (or)
You can group symbols and operators with parentheses.
#elif is equivalent to using:
#else #if
But using #elif is simpler because each #if requires a #endif, whereas a #elif can be used without a matching #endif.
See #if for an example of how to use #elif.