WordPress at a glance
action is not described

rest_insert_user action-hook . WP 4.7.0

Fires immediately after a user is created or updated via the REST API.

Usage

add_action( 'rest_insert_user', 'action_function_name_1854', 10, 3 );
function action_function_name_1854( $user, $request, $creating ){
	// action...
}
$user(WP_User)
Inserted or updated user object.
$request(WP_REST_Request)
Request object.
$creating(true/false)
True when creating a user, false when updating.

Where the hook is called

WP_REST_Users_Controller::create_item()
WP_REST_Users_Controller::update_item()
wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php 535
do_action( 'rest_insert_user', $user, $request, true );
wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php 667
do_action( 'rest_insert_user', $user, $request, false );

Where the hook is used (in WP core)

Does not used.