https://interactivebrokers.com/en/index.php?f=1595&p=calculations It's described under "Calculations":
Here is a related thread about LFIN, which includes some more articles about short positions in stocks which are halted: https://www.elitetrader.com/et/threads/lfin-and-shorting-to-zero.320419/ Tags: LFIN Longfin Northern Rock Bill Alpert short zero bankrupt halt halted