This is the Exchange 2013 product level required for co-existence with previous versions of Exchange, being Exchange Server 2010 SP3 or Exchange Server 2007 SP3 rollup 10.
- Includes Address Book Policy Routing Agent (info);
- Allows group memberships to be managed by groups (again, as it was possible in exchange 2007 but not in exchange 2010);
- Access to Public Folders you have added as favorites via your favorites menu either in Outlook or Outlook Web app (still no regular public folder tree access though);
- EAC has been enhanced and now includes Unified messaging management and migration options;
- Many probes, monitors, and responders have been updated and improved over the RTM release;
- Get-HealthReport cmdlet has been streamlined and its performance has been optimized;
- Supports the Exchange Server 2013 Management Pack for SCOM 2007 R2 and SCOM 2012 (due at a later date);
- High Availability changes
Note that CU1 includes a schema change. Like Service Packs for earlier versions of Exchange, the Cumulative Update is indeed cumulative (hence the size of 1.3 GB) and you can install it directly, i.e. no need to install RTM first. Also, once installed you can't uninstall CU1 or any of the installed roles. The order of upgrading servers doesn't matter, unlike with earlier Exchange versions.
You can download Exchange 2013 Cumulative Update 1 here.