Edward,
From the SRND 7.x, "Synchronization Mechanis" section:
"When synchronization is enabled for the first time on a Unified CM publisher server, user accounts that exist in the corporate directory are imported into the Unified CM database. Then either existing Unified CM end-user accounts are activated and data is updated, or a new end-user account is created ..."
https://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/directry.html#wp1045284
Hope it helps.