Let's say that your margin requirement tonight is $100,000. You have to have at least $100,000 in your account. Your FCM, has to set aside $8000 of their capital toward a risk deposit.
https://www.gpo.gov/fdsys/pkg/CFR-2016-title17-vol1/xml/CFR-2016-title17-vol1-sec1-17.xml
Minimum financial...