ONTd is the base token for uTrade on Ontology blockchain.
The native token of Ontology is ONT, however that token cannot be broken down into fractions (decimals). This means you can never have less than 1 ONT. This makes it difficult to trade tokens which have a value that is less than the price of 1 ONT, or to charge a fee less than that value.
To solve this issue, Ontology has created the ONTd token which is pegged 1:1 with ONT, but it has the ability to be broken down into decimals. So when you're done trading, you can swap your ONTd back into ONT. The only thing to keep in mind is that you will not be able to swap fractions of ONTd back into fractions of ONT.