This link nicely summarizes US situs assets (key points pasted below *).
As an aside from the main topic: I own Fundsmith which is a UK open ended investment company. I find it interesting that UK HMRC (see ** below) has an exemption from IHT. On the other hand US IRS views US mutual funds, which I guess are a similar concept, as US situs assets and subject to US estate tax.
( UK rules about domicile are all currently under review and will change in April 2025 , tbc if this remains the same…)
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(*) HSBC Investing in the U.S. While Managing the U.S. Estate Tax Risk
*What is a “U.S. Situs Asset”?
… Certain classes of assets are specifically exempted from the definition of a U.S. situs asset, and are therefore not subject to U.S. estate taxes upon the death of a foreign person. For example, cash held in a deposit account (e.g., checking, savings, CDs) is exempted from this definition. Similarly, U.S. Treasury Bonds and corporate bonds of U.S. publicly traded companies are exempted.
With regard to a typical investment portfolio, the types of financial investments which are subject to the U.S. estate tax include individual U.S. stocks and U.S. pooled investment vehicles (e.g. U.S. mutual funds, U.S. ETFs, and similarly structured U.S. vehicles). …
It is also important to dispel the misconception that U.S. stocks owned in accounts outside the U.S. are not subject to the U.S. estate tax. As discussed above, this analysis is based on the type of investment within an account, and not where the account itself is physically located. An individually-owned account in Switzerland with shares of individual U.S. stocks will also potentially be subject to the U.S. tax upon the death of the individual foreign owner. "
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(**) UK HMRC
" When someone living outside the UK dies
If your permanent home (‘domicile’) is abroad, Inheritance Tax is only paid on your UK assets, for example property or bank accounts you have in the UK.
It’s not paid on ‘excluded assets’ like:
** foreign currency accounts with a bank or the Post Office*
** overseas pensions*
** holdings in authorised unit trusts and open-ended investment companies"*