Exchange 2010 SP2 and Address book policies
Exchange 2010 SP2 comes with a solution for something that has plagued Exchange administrators for years – the ability to quickly and easily segregate the Global Address List (GAL) and present a tailored view to respective users. Although, there have been different solutions to accomplish GAL segmentation but never been suggested by Microsoft. However, in release of Exchange 2010 SP2 it has been made a cornerstone feature which termed as Address Book Policies (ABP) which enables Exchange administrators to quickly and easily separate address lists for each organization. 
An ABP must contain the following:
- - One Global Address List.
- - One Offline Address Book.
- - One room list. This is required in an ABP. Even if you don’t use these in your company.
- - One or more address lists
ABP in Exchange SP2 are completely Microsoft recommended practices and methods which should encourage you to setup your Exchange environment and solution for the long term benefit. Having these practices ignored in your Exchange SP2 deployment means that you can run into problems and issues which may not be consulted with either Microsoft or the solution vendor. So, the only remedy for your deployment is to be exactly in accordance with Microsoft guidelines or using Microsoft approved and validated solution for the Exchange just like MachPanel.
Image Credit: Microsoft TechNet Community