It is a LEGAL term, an accurate and legal term for someone illegally in the country.
“Alien” is a non-US citizen.
“Resident Alien” is a legal alien that is residing in the US.
“Illegal Alien” is an alien that is here illegally.
“Undocumented immigrant” is a political term.
What are we going to do next, airbrush cigarettes out of old films. . .wait. . .what. . .
Actually the term “Resident Alien” is not used. The term that is used is “Legal Resident”