Total BTCUSDLONGS minus the BTCUSDSHORTS in order to give you the total outstanding BTC margin debt.
ie: If there are 50,000 BTC longs, and 40,000 BTC shorts, then 50,000 has been bought, and 40,000 sold, leaving us with 10,000 BTC net remaining to be sold to give us an overall neutral margin position.
That isn't to say that the net margin position must move towards zero, but it is a sensible reference point, and historical net values may provide useful insights into the current circumstances.
(Anyone know what category this script should be in?)