Nested Applications Explained
An end user logs in to a physical endpoint machine and uses the Horizon Client to connect to a Windows remote desktop, which can be a virtual desktop or a published desktop. Inside that remote desktop, he then launches the Horizon Client and connects to a Horizon published desktop or applications from this remote desktop session. This setup is called nested mode because the Horizon Client connects to a remote desktop that also has Horizon Client installed.
In this case, we have thin clients on mobile carts to use in hospital rooms. In order for to access the apps he needs, some of them will not run on Windows 10, he will use nested mode to access those application published from an RDSH server on an instant clone Windows 10 desktop. Those hospital applications are available inside the virtual desktop via nested mode.