A call leg is a logical connection between two telephony devices, regardless of their type. Call Leg Media Quality Report is dedicated to the quality of the communication channel between the Lync Mediation Server role and an external PSTN gateway (or an external Audio/Video Multi-Point Conference Unit (AVMCU)).
Quoting the Microsoft Support post at http://support.microsoft.com/kb/2798143:
"An update is available that enables you to use a Call Leg Media Quality Report in a Lync Server 2013 environment.... The report also enables you to identify and resolve issues of audio quality that are caused by media server connection issues."
When we open the Call Leg Media Quality Report page, we have a list of connections between our Mediation server and the other media servers. In the following screenshot, we have two SIP trunks that connect Lync to two Cisco Unified Communications Manager (CUCM):
On the right-hand side of the Call Leg Media Quality Report...