I think Canon have FEC(Flash Exposure) because previously their Consumer Class Flash like 380EX do not come with any manual Flash control. Therefore the EOS 50 Class and above have a FEC function to alter the flash Power.
Alternatively, some third party external Flash have ISO settings on the flash for both manual and auto mode. Therefore, if you use it on a camera body without FEC, For example the Rebel Class, you can set the Flash to Auto and adjust the ISO setting to offset the flash power.
For the previous Canon flash system ( ie the ATTL EZ flash systems on EOS 5, 10, etc systems ), the flash overrides the camera. In other words, it is NOT additive. It would be quite logical for Canon to continue this behavior in the ETTL EX flashses, but I don't know for sure since I don't have a 550EX to test it with.