What makes a CDAF lens?

I had an unanswered question earlier that came up concerning the general relative slowness of the Oly 4/3rds lenses' AF on the OM-D E-M5 -- What makes a lens "CDAF enabled" or "suitable for CDAF bodies"? I thought the CDAF/PDAF stuff was all about the body's sensor, electronics, and firmware. I'm not sure where the lens might come into it, doesn't the firmware just fire the lens focus motor for a step or two, re-evaluate the image, and repeat until lock?

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Lester