WordPress at a glance
filter is not described

the_sites filter-hook . WP 4.6.0

Filters the site query results.

Usage

add_filter( 'the_sites', 'filter_function_name_2673', 10, 2 );
function filter_function_name_2673( $_sites, $this ){
	// filter...

	return $_sites;
}
$_sites(array)
An array of WP_Site objects.
$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 310
$_sites = apply_filters_ref_array( 'the_sites', array( $_sites, &$this ) );

Where the hook is used (in WP core)

Does not used.