A CrossBox Server is used to facilitate building and debugging Elements projects on operating systems other than that used by the developer. For example, a developer working on Windows might want to debug a Mac or Linux app, or a developer working in Fire on Mac might want to run an application on a Windows or Linux host for debugging.
CrossBox 2 no longer requires running a custom app on the target machine, and instead uses SSH. (SSH is available on Linux and Mac by default, and for Windows we will be providing a light-weight SSH server you can run, in Elements 10.)
In the long run (Elements 10 and later), we will expand CrossBox 2 to facilitate remote debugging for all target platforms (e.g. including .NET and Java applications), as well as have it replace the use of CrossBox 1 for Cocoa projects.
CrossBox in Fire
A CrossBox 2 connection is required for running and debugging both Windows or Linux applications, since neither can be run locally on the Mac, for obvious reasons. Of course this Windows or Linux machine can be a dedicated PC or server, or a Virtual Machine running inside Parallels or VMware on your Mac.
Island applications can be built locally on Mac, without the need for a remote connection, regardless of target platform.
CrossBox in Visual Studio
CrossBox 2 is (currently as of Elements 9.0) used only for Island projects, to run them on Linux or Windows. Of course Island Windows applications can be run locally on the machine with Visual Studio, so the use of CrossBox 2 is optional for that scenario. For Island Linux applications, a CrossBox 2 connection is required for running and debugging.
Island applications can be built locally on Windows, without the need for a remote connection, regardless of target platform.