CL.BZ is the intercommodity spread (ICS) symbol. Intercommodity spreads are the direct-routed (e.g. NYMEX) guaranteed combos natively supported by the exchange. They are only available for a few specific combinations of futures. ICS orders are not supported in paper accounts.
?
BZ, CL is the smart-routed spread symbol. Smart-routed futures spreads are generally non-guaranteed because they are not natively supported by the exchange but are created by IB. Smart-routed spreads are generally available for many different future combinations.?
?
http://interactivebrokers.github.io/tws-api/spread_contracts.html#bag_fut
