US Single tock Futures (SSF) are a hybrid product, regulated jointly by the SEC and CFTC and allowed to be carried in either a securities account or commodities account.  IBKR elects to carry all SSFs in the security side of an account as this is the only way that margin offset can be provided against other security products (i.e., stock, options). 

US SSFs are listed at the OneChicago exchange and are cleared through OCC.