When you have a margin loss, there is two calculations necessary.
One calculation is that you are loosing a certain amount of coin that has a value on the day you lost it.
So 0.001 BTC is worth 45 USD on the day you have the margin loss charged to your account. That is the Margin Loss. (-45 USD)
You purchased these 0.001 BTC 2 months ago when it was worth only 30 USD. So in addition to the loss you have generated a capital gain for these BTC of 15 USD. This is the margin delta (+15 USD). The delta can be positive or negative depending on whether the price rose or fell between the purchase and the margin loss.
This FAQ article might be useful as well: How does the CoinTracking purchase pool work?
In terms of tax law, these deltas fall under the private sale transaction. This means that personal income tax is due on the deltas if the holding period of one year is not reached.