CADs have to be in close proximity to the magician in order to work properly. Please take note, that I say "in close proximity." Orbital type CADs are possible.
CADs are created by inscribing an object with formulae designed around the eidos of a magician and their magic circuits. The whole process is highly complicated and only trained CADsmiths know the exact details. (read: I can't come up with a good explanation so hopefully this cop-out explanation will suffice! Sorry! I haven't had time to focus on the technical history on CADs since I was too busy focusing on the technical aspect of magic itself.)
CADs are initially issued by the magic school where the young mage is enrolled in. Before enrty into the school, all new mages must undergo a physical examination. This is partly for health reasons but the data acquired from the examinations will be used to sync up their CADs with their biosignatures. Remember the magic class I showed earlier? All the students there were using Bracelet CADs issued by the school in order for them to participate in class. After their first year of school, students have the option of designing and customizing their own CADs (under the supervision of the school of course). Graduates and working mages have their CADs fixed, customized, and resynced through the Academy branch offices found in each city.