That wording is used both by the IRS and USCIS. Non-GC holders are non-resident aliens, but some visas are "dual intent" which allow you to become a resident alien/permanent resident/green card holder. For the IRS being a resident alien means that when you get kicked out of the country after (iirc) living in here for 7 years you have to pay taxes on any assets you own in the US as if you had sold them, just like Americans have to when renouncing their citizenship.

Edit: Quick googling to double check myself and it seems you're right in that USCIS indeed doesn't use that wording, which I must be getting confused with some other term they use in some form, or some other agency, or I'm just plain wrong and got the IRS wording internalized with something else.