WordPress at a glance
filter is not described

is_multi_author filter-hook . WP 3.2.0

Filters whether the site has more than one author with published posts.

Usage

add_filter( 'is_multi_author', 'filter_function_name_3630' );
function filter_function_name_3630( $is_multi_author ){
	// filter...

	return $is_multi_author;
}
$is_multi_author(true/false)
Whether $is_multi_author should evaluate as true.

Where the hook is called

is_multi_author()
wp-includes/author-template.php 536
return apply_filters( 'is_multi_author', (bool) $is_multi_author );

Where the hook is used (in WP core)

Does not used.