Source

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

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

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

		$this->cap_key = $wpdb->get_blog_prefix( $this->site_id ) . 'capabilities';

		$this->caps = $this->get_caps_data();

		$this->get_role_caps();
	}