回复：AC21是一个法案或者是人道的政策，不是什么需要FILE的东西吧

You are right. The so called "file AC21" means some people send a explanation letter to USCIS to address their change of job is based on AC21 rule. Although such a letter is not required and not necessary, these people just feel unsafe in their mind when they change their employer, so they "file AC21".