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To figure out how much margin a trade is going to require, you use the Analyze tab in TOS. This is where the slices come in. You add the simulation of the trade you want to do, and it will show the PM requirement. The rules are explained at: https://tlc.thinkorswim.com/center/faq/Portfolio-Margin You need to short a box spread to increase the cash vehicle at the expense of some margin. Unless you intend to withdraw the borrowed funds out of your TDA account, a box spread won't use much margin. Again, you can model it in the analyze tab.

I traded T-Bills in TDA and with Portfolio Margin. If I buy $50,000 worth of T-Bills then my "Stock Buying Power" goes down by about $500, because it is low-risk asset, but my "cash and sweep vehicle" was decreased by $50,000 .

In TDA, you get charged margin interest if your cash & sweep vehicle gets under zero. To open trades, what matters is the available funds for trading. Different instruments, and different combinations of options have different impacts on the available funds for trading, what we would refer as margin. For instance, to buy one thousand shares of SPY now, I wouldn't need $443,800, just $66,568.50, as you can see in this screenshot I just took from my account. If you borrow with a box spread, that increases your cash vehicle at the expense of your margin, and if you lend with a box spread, you lose cash vehicle but not much margin. Box spread are a great way to manage those two buckets so that you don't need to spend expensive margin rates, just borrow or lend from the options market.

I've never done it so I have no direct knowledge. From the conversation, I go the impression that the cash and sweep vehicle could be negative, possibly identifying how much margin you are actually using. The problem with quantifying PM margin is that the actual margin applied to a trade is figured with a set formula, but the variables to said formula can change on how risky the broker feels about the underlying at that particular instant. This whole conversation has made me want to look into it sufficiently to understand it, kind of like balancing your check book, I've done it for over 50 years but I've never found a mistake by the bank, but I'm going to still do it.

If you have cash $100,000 and you will buy stock worth $2,000,000 , your cash and sweep vehicle will become -$1,900,000 and you will be charged an interest for $1,900,000 amount.

I would love to see those statements (happy to send email), as this is vastly different than my experience and anything I've ever seen (and doesn't even make sense -- that's a free loan of millions).