Source

File: wp-includes/class-wp-customize-control.php

public function get_link( $setting_key = 'default' ) {
		if ( isset( $this->settings[ $setting_key ] ) && $this->settings[ $setting_key ] instanceof WP_Customize_Setting ) {
			return 'data-customize-setting-link="' . esc_attr( $this->settings[ $setting_key ]->id ) . '"';
		} else {
			return 'data-customize-setting-key-link="' . esc_attr( $setting_key ) . '"';
		}
	}