Source

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

public function for_site( $site_id = null ) {
		global $wpdb;

		if ( ! empty( $site_id ) ) {
			$this->site_id = absint( $site_id );
		} else {
			$this->site_id = get_current_blog_id();
		}

		$this->role_key = $wpdb->get_blog_prefix( $this->site_id ) . 'user_roles';

		if ( ! empty( $this->roles ) && ! $this->use_db ) {
			return;
		}

		$this->roles = $this->get_roles_data();

		$this->init_roles();
	}