A 706-NA is one of those forms that you have never really heard of. Or at least you can get by without hearing about it until, for example, someone dies. And even then you may not hear about the form. You may just understand from some professional that you may have to file something with the US. That the deceased had US assets at date of death.
Fun stuff you say. Yes, not fun at all. Even for me as I go through these forms and assemble a pile of what you had in your possession on your date of death, even I think wow this sucks. But whatever, getting your stuff in order does provide clarity, so that's how I look at it. Anxiety, be gone, you have met the filing obligations of the IRS and did it with grace and elegance - hoping at some level this was accomplished, though highly unlikely.
Anyhow, how it works is if you owned $60,000 USD in US situs property and are not a US citizen or US person then voila you need to let the IRS know your business. Yup an inventory of what you owned and what proportion of that inventory was US situs. Send it all in. Most likely no tax is due but still you gotta let them know that you left this earth with some kind of souvenir from the good old USA.