The most common way for trading commodities is to buy or sell a futures contract. The price of a commodity futures contract is standardised, meaning the underlying instrument’s quantity (pound, ounce, barrel, etc) is predetermined and appears the same for all market providers.
A futures contract also obligates the holder to buy or sell a commodity at a predetermined price on a delivery date in the future.
In CFD trading, once a commodity futures contract expires, a trader can either close the trade and open a new trade, or alternatively, allow the contract to roll over to the next month (if possible).