请问这句英语中最后的关系从句中的关系代词that指的是什么?

题目

请问这句英语中最后的关系从句中的关系代词that指的是什么?
This marks one of the biggest changes in the Internet's domain name system,the addressing system of the Internet,that has ever occurred.

答案

This marks one of the biggest changes in the Internet's domain name system,the addressing system of the Internet,that has ever occurred.直译：这标志“互联网域名系统 亦即互联网编址系统的”最重要的变化...