I think that in order to decide what it is, you first have to decide what it isn't. Using that logic I conclude that those accounts are not cash and not freely convertible, by default they more closely resemble capital investments and the tax treatment of them is probably no different from that of any other form of markets based investment that incurs a capital gain. If the account was real estate, it also could have a valuation dated at year end, making past profit income free of tax under Por 162. But because you are unable to ring fence that income, the following day, on 1 January 2024, the value of the property increases and you have commingled funds.