It should be the Fabric Manager Server license and allows access to certain Cisco Fabric Manager features. It's required for features such as historical performance monitoring, Cisco Fabric Manager java client login to a remote server, report generation and viewing with the Web Client.
I've found the following document on the Nexus 5000 Fabric Manager Server which might give you a bit more information.
The unlicensed Cisco Fabric Manager can only monitor and configure one fabric at a time. You must use the Open menu to switch to a new fabric, which causes the application to stop monitoring the previous one and to rediscover the new fabric.
I only have a NX5K i don´t have MDS, so that I thinks that unlicensed Cisco Fabric Manager is sufficient.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...