Principal place of abode in the US

See page 3 of F8812 instructions - residency requirement   https://www.irs.gov/pub/irs-pdf/i1040s8.pdf

I would say check out Pub 17 Part 4 page 105

I hate it when they make these forms so complicated 

Line 13
Box A
Check box “A” if you (or your spouse if filing jointly) had a principal
place of abode in the United States for more than one-half of 2021.
This means your main home was in the 50 states or the District of Columbia for more than one-half of 2021. Your main home can be any
location where you regularly live. Your main home may be your
house, apartment, mobile home, shelter, temporary lodging, or other
location and doesn’t need to be the same physical location throughout
the tax year. You don’t need a permanent address. See Box B, later, if
you were a bona fide resident of Puerto Rico in 2021.
If you are temporarily away from your main home because of illness, education, business, or vacation, you are generally treated as living in your main home