Many digital assets that Binance.US offers can be paired with USD. In other words, you may use a USD balance already deposited into your account to purchase them.
However, some assets do not have a USD pairing available. In these instances, you may select another asset pair to trade to, including a USD pair with an asset that can be traded or converted for the original asset.
To see an asset's full list of pairings, type the asset's name in the search function within the Trading section of the app or webpage.