Annoucement Article: Here

Changes include ( comments in Italics are my own):

Fix: "Duplicated handles are not managed correctly and cause the virtual application to crash." This is believed to be a client fix, affecting both the Windows Desktop and RDS Clients. I have no information on what application(s) may have been affected.

Fix: "The life cycle of the effective group registry entries are not maintained correctly."  This also sounds like a client side fix, this time involving Connection Groups.,

But wait, there's more?

While these are the only changes identified by Microsoft, we do have new builds for the sequencer and the server. I am guessing that these are just automatic rebuilds at this point.  In all likelihood nothing has changed except that it has been rebuilt.  But we really don't know.

Sequencer:  The sequencer is always released as a full installer, never with an updater.  To install you should really start with a clean image (yes, it will work if you first uninstall the old sequencer, reboot, and then install the new. But you really shouldn't do that!). No changes in functionality or fixes have been detected in this build to date.

Server:  The binary files for all three server components (management, publishing, and reporting) are updated.  Some of the dependent components not written by the App-V team remain with their old dates. Database Schema version remains at 2 for the Management database and "5.1.85.0" for the Reporting database. No changes in functionality have  been detected in this build to date.

This hotfix is a direct download of the components that you are interested in, including other MDOP components.  No email required on this one!

Note: Windows 10 version 1607 and above, and Server 2016 and above, clients do not install these hotfixes as standalone installers any more.  The fixes would be applied to, and delivered by, operating system updates delivered via normal channels (e.g. "Windows Update").  Microsoft tells us that fixes should generally appear first in the 10/2016 branches before releasing the 5.1 branch; 

NOTE:  App-V Hotfixes always include prior fixes on the same release.  In other words, you only need to apply the 5.1 RTM plus the latest hotfix (5.1 HF9 in this case) and it includes other prior hotfices (5.1 HF1 through HF8 in this case).