Changes include ( comments in Italics are my own; we are awaiting additional details/understanding  on most of these changes):

• The lock on the VFS folder was not getting released when the user logs off from the client service.

I think this is mostly relevant on an RDS Client. This  is likely a catch up with a fix in 5.0 SP3 HF2 which stated: When you run a virtual application and Copy-on-Write is invoked to redirect file writes, the App-V client keeps an open handle to the COW area on the user profile, even after the user logs off.  This interferes with profile cleanup issues such as profile deletion.  This is resolved.

• Microsoft Office Click-to-Run apps can't be opened from SharePoint in Internet Explorer. Embedded Office Click-to-Run apps may crash.

A continuation of a 5.0 SP3 HF2 fix involving Office C-T-R and SharePoint.

• Press and hold (or right-click) app on taskbar, the app icon becomes blank.

I have heard from customers with a blank icon scenario; it is likely that this is just one manifestation that they fixed.

• Provide mitigation for apps which don't handle long paths correctly

Need more detail on this.  Might be a fix for a lot of packages that required PVAD or we couldn't get to work.

• Enable users to create multiple instances of Office 2016 Click-to-Run apps.

Curious to understand details on which scenarios are now supported that were not previously.

• Re-enable APP-V 5.1 Management server address "https://localhost:<port>/packages" and "https://localhost:<port>/packagesummary".

Does not affect production use of the server.  However these are links I would keep on a administrator's test page to check that the management server is working prior to 5.1.  The key to using these links is that you either had to run them from a Publishing server or  add your machine to the publishing server list in the database (even if it doesn't do publishing).

• The Confirmparameter is ignored when the Groupsparameter is null for Grant-AppvServerPackage command. Use the newly added Forceparameter to avoid the "Confirm Action" dialog.

Would probably not affect normal usage of the server from the GUI?  My money is on this affecting a customer that wrote their own automation tool and was skiping the  management  console.

App-V Hotfixes always include prior fixes on the same release.  In other words, you only need to apply the latest hotfix (5.1 HF2 in this case) and it includes other prior hotfices (5.1 HF1 in this case).

Client is version 5.1.101,  Server is 5.1.102.  I believe that you may apply these independently.  The Server hotfix appears to only apply to the Management Server and would not need to be applied to Publishing and Report servers.  THe KB reports that you must reboot after applying the patch, so a maintenance window is needed.

Download from Microsoft Support Downloads: KB3139245 https://support.microsoft.com/en-us/kb/3139245