I generally hear a vague rule that someone on visitor visa can't stay for more than 6 months in a year, but don't know the specifics of the rule - anyone know the details of it? Like calendar year or any consecutive 365 days, is it like a 180 days clock etc. We have a parent who is staying 170 days this year June-Dec. Is it possible for them to enter the country again in May 21 for 15-20 days on a short trip?
Yes, they can enter.
Some officers really do not like it if the visitor stays for 180 days. Its more like a limit that you do not want to reach quite often.
Is their I94 still valid? Did you have it extended?
Not a vague rule, probably you will find it on uscis website as well. max 180 days in consecutive 365 days. clock doesnt reset on 31 dec or any other day for that matter.
The usual rule officers follow is - you should stay outside the country atleast the number of days you stayed in US in your last visit. Remember- it’s a visitor visa. Generally speaking you won’t be staying a country more than you do in your home country. I have heard from my colleague’s parents that the officer asked them why they are visiting again within a short span. Bottom line - it’s at the discretion of the CBP officer.
Yes. Entry is at officer's discretion tho