WordPress at a glance
action is not described

pre_get_sites action-hook . WP 4.6.0

Fires before sites are retrieved.

Usage

add_action( 'pre_get_sites', 'action_function_name_6456' );
function action_function_name_6456( $this ){
	// action...
}
$this(WP_Site_Query)
Current instance of WP_Site_Query (passed by reference).

Where the hook is called

WP_Site_Query::get_sites()
wp-includes/class-wp-site-query.php 241
do_action_ref_array( 'pre_get_sites', array( &$this ) );

Where the hook is used (in WP core)

Does not used.