One unusual aspect of Stir Options is the choice of a variety of different underlying futures contracts for a given expiry date.

The norm in other exchange traded options, for example, equities, is that the option holder always exercises into a spot position.

In the underlying with stir options, there is a choice to trade standard options, which exercise into the adjacent futures contract, or mid curves, which exercise into a futures contract further along the forward curve.

For example, if I buy a standard September call, then I would exercise into a long position in the September futures contract that is the futures contract, whose period start date matches the expiry date of the option.

Alternatively, if I buy a September one year mid curve call, then on exercise, I would get a long position in the September futures contract one year forward from the expiry date.