Trading Credits are designed to be automatically applied to your derivative trades to offset your margin. Trading Credits apply to all derivative trades with the same margin currency (i.e. USDT credits will only cover USDT settled contracts etc.). They will also work on other margin currencies, however only when using Multi-Asset Margining. For more information on this please read more here. They do not apply to Spot positions.
Example: If you have 100 USDT Trading Credit and you open a position requiring 150 USDT in initial margin, your Trading Credits will automatically cover 100 USDT of that margin. This means you would only need to use 50 USDT from your own funds to open the trade.