ja, genau(thomas, bist du tho_mas auf LL?)
that the Contax can write it to the film has nothing to say here as the film back has the 9 contacts the camera has. The Phase backs with Contax mount have these 9 contacts as well so the aperture (shutter speed and ISO, too, of course) is stored in the EXIF.I don't yet know if the Contax stores more than EXIF in the Sinar back's files' EXIF, but I presume it stores shutter speed, aperture, ISO, and various strings, since it is able to write these values to a film (!), and has a full data connection to the back. It should be verified though.
Somewhere I read (or heard?) that there was knowlegde exchange between Phase and Contax in the construction phase of the Contax. Don't know if anything of this is true... anyway the Contax with a Phase back is a unity, it works with a digiback more or less like with a filmback (just slower... depending on the back); e.g. you set ISO at the back and the Contax metering considers the ISO (which is trivial but obviously not possible with all cameras). And no cable needed of course.