It aggravates people because it is used on purpose to confuse. There are legal and illegal immigrants. We welcome legal immigrants. Always have. It is illegal immigrants that are the issue. This is why we have two terms; immigrants and illegal aliens. It's pretty simple and is considerably more accurate.

I have no idea what an undocumented migrant is. This term is used often when talking about illegal aliens yet many, those in school in particular, have plenty of documentation. The terminology simply makes no logical sense within the English language.