TDA has a third type of margin account besides regular and portfolio: https://tlc.thinkorswim.com/center/howToTos/thinkManual/Miscellaneous/RBC-Margin Does anyone know the difference between this and portfolio?
Yeah. It reads like where with a portfolio account you have to actually go negative to get a margin call, this RBC deal has to show only the POTENTIAL IN THE FUTURE to go negative, at which point they will margin call you and close your positions, even though you still have plenty of margin for the current position in the account. Some formula they use to "estimate the potential range" of an instrument on that given day. So they liquidate you if it looks like you might go negative, but aren't actually negative. A weird deal.