FA02/SCH26/PARA11 sets out what is meant by the underlying subject matter (USM) of a relevant contract.
See examples at
CFM13098a
The qualification that the USM of a future is the property
whose price and delivery date is agreed at the outset of the
contract means that something does not become the USM of a future
just because it is handed over (or might be handed over) at the
maturity of the contract. In the first example at CFM13098a, the
company receives cash in settlement of the sugar futures. This does
not mean that cash is an USM of the contract. It is a means of
payment. The same is true even if a company settles a contract by a
payment in foreign currency, or even by handing over a parcel of
bonds or shares to the value of the agreed price.