Let's say you invest $1,000 in a BUY trade with the Stop Loss set to the maximum allowable loss of 50%. This means that the trade will close if it reaches a loss of $500.
If you wish to extend the Stop Loss to 100% once the trade is open, an additional $500 will be allocated from your available balance to the invested amount. It brings the total invested amount to $1,500, and the Stop Loss to $1,000. The additional $500 is what we call the 'maintenance margin'.
The maintenance margin is always based on the initial invested amount. If you have already increased the size of the trade you wish to update, via previous Stop Loss extensions, the gap between the current invested amount and the 50% maintenance margin will be larger.