WordPress at a glance
filter is not described

allow_subdirectory_install filter-hook . WP 3.0.0

Filters whether to enable the subdirectory installation feature in Multisite.

Usage

add_filter( 'allow_subdirectory_install', 'filter_function_name_2797' );
function filter_function_name_2797( $allow ){
	// filter...

	return $allow;
}
$allow(true/false)
Whether to enable the subdirectory installation feature in Multisite.
Default: false

Where the hook is called

allow_subdirectory_install()
wp-admin/includes/network.php 61
if ( apply_filters( 'allow_subdirectory_install', false ) )

Where the hook is used (in WP core)

Does not used.