ZCS version 6.0.x
ZCS 6.0.x uses a ranking table to place more frequently emailed contacts at the top of the autocomplete list. Only addresses with successful deliveries move up in rank.
ZCS version 5.0.x
The 5.0.x web client will do its own search in the addressbook, then query the server to autocomplete against the GAL. The result of the GAL autocomplete is appended to the end of the autocomplete results. Email addressees are saved to Emailed Contacts if the "save to emailed contacts" user preference is checked.
Also in 5.0.x, the web client caches the entire addressbook. If there are new contacts not loaded by the web client, then it will not show up in the autocomplete list. The web client caches the previously used autocomplete result if the previous query with the same prefix resulted in a smaller number of hits.