Super — compulsory, and unsettled
Superannuation Guarantee contributions are paid by your employer by law. You cannot decline the account, which is what separates this from every other problem wrapper in expatriate tax.
Its US characterisation is genuinely open — a foreign grantor trust, an employees' trust under §402(b), or a pension within Article 18. There is no ruling settling it, and the reading adopted in year one shapes every year after.