US rules require an fcm to hold customer futures money in segregated accounts and to top up any shortfall from its own capital daily. Unlike securities accounts, there is no SIPC-style insurance behind it: segregation is the protection.
Segregated status also means that in a broker bankruptcy customer positions and collateral can be ported to another FCM rather than becoming part of the estate — as long as the money is actually there.
Example: an FCM reporting $1.2 billion of segregated requirement and $1.26 billion held has $60 million of excess segregated funds. A negative number is the warning sign regulators look for.
Related: fcm, clearing-house, cftc, introducing-broker, guaranty-fund