When I restart some of my access points, after they restart I'll sometimes see an "Upload application signature" under Device Update Results. Just curious as to what the application signature is and what its purpose is.
Go to your Hivemanager account and have a look at Dashboard / Applications. Notice the list of applications identified in your Network? Well, your Access Points are able to identify them thanks to the loaded "Application Signature" file.
The message you see means that your APs have just uploaded their application signature files, and thus are now able to identify all applications that are currently supported by Aerohive.
Note that there are slight differences depending on hardware model and firmware version. But in general, this is how it works.
And last: Every Firmware update usually comes with an updated Application signature version, but sometimes their update has to be triggered manually.