The DTCC requirements are all about protecting brokerages (and other counter parties) from each other. Basically (and I’m not an expert in this) it’s a risk sharing scheme where if Robinhood (or any other broker) goes under all the other members cover the positions.

I don’t think it has anything to do with margin vs non-margin accounts for instance. It’s just a formula where you split up the outstanding shares by the VaR as I’d get a capital requirement.