Fictitious Credit

If you deposited money with Ebenezer Scrooge or Gordon Gekko.

Outside of literature and film, it's a component of a short sale.

A short sale involves borrowing a security so you can sell it for cash. You hope the security will go down in price. When it does, you buy it back at the lower price, return the security you repurchased to its original owner, and keep the profit. You sold high and bought low, keeping the difference in-between.

When you make a short sale, you end up with cash in your account. Eventually, you have to use that cash to buy back the security in order to close out the trade. So when you make the short sale, you've created a margin requirement.

You have the money, but it's spoken for. It has to be used to buy back the security you've shorted. You can't actually use the cash for anything (other than closing out the short). It's not a real credit, like you'd have if you put your own unencumbered cash in the account. It's fictitious credit.