When an application (RemoteApp, RDP or full desktop) is launched, a list of candidate servers for the application will be created.
There will also be a validation to check whether the server is reachable (Connectivity to the TCP port specified for this server, by default 3389).
If this is not the case, the application server will not be held as a valid candidate. As a result, no users will be connected through Parallels Secure Workspace to that application server.
A common issue when using an RDS broker on the Microsoft Windows back end: User ends up on different RDS host than the one specified for the application in Parallels Secure Workspace