
Explanation:
http://technet.microsoft.com/en-us/library/jj204672.aspx

Client Interoperability in Lync 2013
This topic discusses the ability of Microsoft Lync Server 2013 clients to coexist and interact with clients from earlier versions of Lync Server and Office Communications Server.Server and Client Compatibility.
The following table shows the supported combinations of client versions and server versions. This table indicates whether sign-in is supported when the client attempts to connect to the server indicated. Lync Server 2013 supports the previous client version. Also, unlike previous releases, Lync Server 2010 supports the new Lync 2013 clients. This allows organizations who are upgrading from Lync Server 2010 to roll out new clients independent of Lync Server upgrades.


Interoperability among Clients
With the Lync Server 2013 release, various client versions can interact seamlessly in both peer-to-peer and conferencing scenarios. This section discusses feature availability when users interact with other users who are using different versions of clients and servers. Peer-to-Peer Feature Support
Peer-to-peer features are supported for users who are homed on different versions of the server and who are using different client versions. The end-user experience and available features are consistent with the capabilities of the user's client and the version of the server the user is signed in to. In other words:
If a user is signed in to Lync Server 2013 with an older client, the user will have the same experience he or she is used to. None of the new features introduced in Lync Server 2013 will be available until the user's client is upgraded.
If a user is signed in to Lync Server 2010 with a Lync 2013 client, any new features not supported by Lync Server 2010 will be unavailable until the user is moved to Lync Server 2013.
The following table compares feature availability in peer-to-peer sessions where the client is signed in to either Lync Server 2013 or Lync Server 2010.
Note:
Lync Web App and Lync 2010 Attendee are meeting-only clients and aren't included in this table.

Conferencing Feature Support
In Lync Server 2013 meetings, conferencing features are supported for users who are homed on different
versions of the server and who are using different clients and client versions. When clients join a Lync Server 2013 meeting, users have access to the features and capabilities shown in this table.

1.In Office Communicator 2007 R2, only desktop sharing (and not program sharing) is available.
2.Lync Server 2013 uses an updated mechanism for uploading PowerPoint files. Lync Web App users who join a meeting that was originally scheduled on Lync Server 2010 can view and navigate PowerPoint presentations, but cannot upload PowerPoint files.
3.If the meeting was scheduled on Lync Server 2013 and PowerPoint slides were uploaded by a Lync 2013 client, Lync 2010 users have view-only access to the slides. Conversely, if the PowerPoint slides were uploaded by a Lync 2010 user, Lync Server 2013 users will be able to view and slides and, if Office Web Apps Server is configured, access new capabilities such as higher resolution display, animations, slide transitions, and embedded video. For more information, see Configuring Integration with Office Web Apps Server and Lync Server 2013.
4.The presence and IM features in Office Communicator 2007 R2 are compatible with Lync Server 2013, but conferencing features are not. During migration from Office Communications Server 2007 R2, Office Communicator 2007 R2 is suitable for presence and IM interoperability, but users should use Lync Web App 2013 to join Lync Server 2013 meetings.
Scheduling Add-in Support
Server support for the various scheduling add-ins is consistent with server and client version compatibility. In general, the following scheduling add-ins are supported on Lync Server 2013. However, previous versions of add-ins do not provide new Lync 2013 add-in features, such as the option to mute all attendee audio and video upon meeting entry.
Online Meeting Add-in for Lync 2013 Provides the same features as the Online Meeting Add-in for Lync 2010, with the addition of attendee mute controls, which allow meeting organizers to schedule conferences that have attendee audio and video muted by default. Administrators can also customize the organization's meeting invitations by adding a custom logo, a support URL, a legal disclaimer URL, or custom footer text.
Online Meeting Add-in for Lync 2010 Provides scheduling for Lync meetings and removes the capability to schedule Office Live Meeting conferences.
Office Communicator 2007 R2 Conferencing Add-in Provides scheduling for both Office Live Meeting conferences and Office Communicator 2007 R2 conferences.
Note:
Live Meeting conferences cannot be scheduled on Lync Server 2013.

Support for Joining Meetings
All of the clients that Lync Server 2013 supports are allowed to join Lync 2013 meetings. Because Lync Web App is a web component of the server, in cases where Lync Web App is used to join a Lync Server 2013 meeting, the newer version of Lync Web App is always used.
Lync 2013 clients can join meetings hosted on Lync 2010 and Office Communications Server 2007 R2 with scaled-down functionality. In-meeting features are limited by the version of the server on which the meeting is hosted.