”Combined Excess Margin”?

I see that number on my Margin Account and I'm wondering if I can use some of that margin even if I don't put much capital on my next purchase.

For example, let's say it says my ''Combined Excess Margin'' is 20k, can I buy 20k worth of VOO with just 1k of capital in my trade (19k borrowed)? Or will I need to do a purchase with at least 10k or something capital (to hit the capital % requirement) and then borrow 10k on the excess margin?

I don't want to touch any of that without knowing how that part works first. Nor do I plan to use that much margin like in my example.

Thanks.