It's the details that are obscure - when or if Twitter ever paid rent directly or if (as some of the other pubs have suggested) it just established this bucket and said withdraw as needed. The setup predates Musk so it's possible there was some mix.
What's the $900,000 amount then? Is that just the limit, how much the bank will pay at most?
It's the max amount that the landlord can claim. It's typically a negotiated dollar amount between Twitter and landlord. Usually put in place at beginning of lease. Pretty much same as a rental deposit, but instead of giving cash, Twitter provide an letter of credit.