This document seeks to address and clarify the small misunderstanding surrounding the GAL (Global Address List) and the OAB (Offline Address Book).
Also, In Exchange 2013, the OAB files are generated and stored in the Organization Mailbox first and later copied to the %Exchange Install Path%\Client Access\OAB\.
CAS server or Client access server was introduced in Exchange 2007.
With Exchange 2010, users/outlook/MAPI clients connect to the RPC Client Access service on the Client Access Server to access mailbox information from a mailbox server.
This situation however is extremely rare since the default maintenance time on Exchange is at AM and updates and downloads are usually performed during working hours.
*** NOTE: The GAL is essentially a real-time / online representation of AD Objects that are Mail Enabled for Microsoft Exchange *** The GAL contains information for all email enabled users, Distribution Groups, and Shared Mailboxes and Exchange Resource Mailboxes.