A Microsoft Exchange Address Book Policy is a feature that allows an administrator to segment an Exchange Global Address List.
Address book policies are especially important in various types of large Exchange organizations. In large companies with thousands of users, each user doesn't necessarily need access to the email address of every employee in the organization. The administrator can tailor an ABP so users are only allowed to view users' email addresses that the administrator -- or management -- deems relevant to their position.
Address Book Policies were first introduced in Exchange Server 2010 Service Pack 2. Though Address Book Policies (ABPs) are exclusive to Exchange 2010 SP2 and Exchange 2013, the actual functionality has been around much longer in the form of a feature known as GAL segmentation (also known as GAL segregation). Address Book Policies further that functionality, though it has been vastly improved and configuration steps have been simplified.