WordPress at a glance
action is not described

wp_initialize_site action-hook . WP 5.1.0

Fires when a site's initialization routine should be executed.

Usage

add_action( 'wp_initialize_site', 'action_function_name_7150', 10, 2 );
function action_function_name_7150( $new_site, $args ){
	// action...
}
$new_site(WP_Site)
New site object.
$args(array)
Arguments for the initialization.

Where the hook is called

wp_insert_site()
wp_initialize_site
wp-includes/ms-site.php 95
do_action( 'wp_initialize_site', $new_site, $args );

Where the hook is used (in WP core)

wp-includes/ms-default-filters.php 50
add_action( 'wp_initialize_site', 'wp_initialize_site', 10, 2 );
wp-includes/ms-default-filters.php 51
add_action( 'wp_initialize_site', 'wpmu_log_new_registrations', 100, 2 );
wp-includes/ms-default-filters.php 52
add_action( 'wp_initialize_site', 'newblog_notify_siteadmin', 100, 1 );