Call it whatever is correct in the parlance of government accounting or practice. However, my reference is to those funds contributed by individuals that will be expecting to get that (their) money back at an appropriate time.

As opposed to the genral fund with is not designated toward individual tax payers and thus available to squander.

No different than contributing to a retirement fund in a corporation. That money is not to be used by the corporation and replaced with an IOU.

I am aware of companies that do siphon off what they have decided are excess funds and used for other than the individuals retirement. Excess is/was the amount of $$ over and above the amount needed to satisfy all retirement payouts. Much different than an IOU as the basic contribution plus earnings is still intact. Making use of the excess....a legal accounting tactic.
I did not say right...just legal tactic.

The government could just as well have had an excess if they monies taken from SS were invested and not squandered......replaced with an IOU!!!
Borrowing money to fund IOU's. How long would that be tolerated in corporate America? Banking America?