The underlying of USOUSD is WTI Futures.
XNGUSD's underlying are Natural Gas Futures
Due to the particularity of the delivery of energy futures, in order to make the trading price smoother and avoid the discontinuity of the trading resulting from the expiry of the position, we use the most recently expired futures contracts, as well as subsequent contracts, to price CFDs in a time-weighted form. Sometimes this CFD pricing method is also known as crude oil spot or natural gas spot, but please note that there is no spot concept in energy delivery.
For example, suppose that the most recently expired futures contract is a June contract (Like WTI June'19), because the price is from the previous month and the second month futures, then the June and July contract-weighted mix is used as the underlying. As we approach the closing date, the weight of the June contract will be smaller and the weight of the July contract will be even greater.