Source

File: wp-includes/class-wp-rewrite.php

public function add_endpoint( $name, $places, $query_var = true ) {
		global $wp;

		// For backward compatibility, if null has explicitly been passed as `$query_var`, assume `true`.
		if ( true === $query_var || null === $query_var ) {
			$query_var = $name;
		}
		$this->endpoints[] = array( $places, $name, $query_var );

		if ( $query_var ) {
			$wp->add_query_var( $query_var );
		}
	}