Most of the organization have Exchange multi-forest environment. Organization could be in multi forest environment because of the merger and acquisition or it could be because of security reason. Auto discover is the new feature introduced in Exchange 2007 and its been carried forward in all the subsequent version of exchange like Exchange 2010 and Exchange 2013.
Below link should give you good understanding on the information about
Hope you got some good understanding on Autodiscover in Exchange 🙂