Film TTL using off-the-film metering of light, and the camera tells the flash 'stop outputting light' at the right amount of light seen at the film plane.
digital TTL does not work that way, the sensor surface is too shiny, so the digital camera instead commands 'preflash' (at a fixed, reduced amount of power) and meters what comes back from the preflash (and not off the sensor) and then sends a command 'flash at 1/n power' (based upon what it read during preflash).
A Metz 54MZ can do BOTH of the above, and it is an SCA compatible flash. A Metz 45CL4 Digital can do both of the above; the Metz 45CL4 cannot do a preflash, so is not compatible with Canon eTTL.