Margin longing/buying of ETH would require you to borrow USD to buy ETH on the order book. Margin shorting/selling of ETH would require you to borrow ETH to sell on the order book.
For a quick reference, you will see your borrowed funding type reflected in your position's "Funding Cost" on the Trading page. If you borrowed USD to fund the position, Funding Cost will accrue in USD. If you borrowed ETH to fund the position, Funding Cost will accrue in ETH.
