Used when computing Net P/L on the Positions page.
Defaults match Interactive Brokers Fixed / IBKR Lite for US equity options:
$0.65 per contract, $1.00 minimum per order
(IBKR docs).
IBKR Tiered: ~$0.15–$0.65 per contract by volume, plus exchange / regulatory fees. Estimated effective ~$0.55/ct, $1.00 min.