I got this setting in my csproj file while using .NET Reflector Pro in Visual Studio to generate PDBs for 3rd party DLLs and debug them.
The Visual Studio hosting process improves debugger performance and
enables new debugger features, such as partial-trust debugging and
design-time expression evaluation. You can disable the hosting process
if you need to.
Here is more information about this host process and debugging: Debugging and the Hosting Process