What is the ($Foo)PS project in my $Foo ATL solution for?

COM supports making interface method calls across two different threads, two different processes or two different machines. This is called marshaling. Two different threads is the most common case, a COM server is often not thread-safe. COM implements thread-safety for such single-threaded coclasses by marshaling the call from the 'wrong' thread to the thread that created the server. Marshaling between processes occurs when you write an out-of-process server. Between different machines across a network is called DCOM.

This is implemented by creating an instance of the interface that looks exactly like the original. But all the methods of the interface are actually substitutes that do the job of the marshaling the call. This is the proxy. On the other end of the wire there's a substitute that looks exactly like the interface but does the opposite job. This is the stub. The proxy and stub work together to create the illusion that you're making a simple method call in your program.

The primary job of the proxy is to serialize the arguments of the method call into a memory buffer or network packet. This can be pretty untrivial, especially when you use pointers to variable-sized structures. COM needs help to get that right and that's the job of your FooPS project. When you run midl.exe on your .idl file, midl auto-generates code from the interface definitions to implement the proxy and the stub. This is quite often good enough but you may need to implement your own if the built-in keywords in IDL are not sufficient to describe your data.

Last but not least, Windows provides a standard marshaller that can marshal simple interfaces. Designed to support the sub-set of COM that's defined by COM Automation. In other words, interfaces that derive from IDispatch and only use Automation compatible types. You only need to get the registry entries right to enable it and don't otherwise need the proxy/stub generated by midl. And of course, if you only make simple in-process calls on one thread then you won't need it either. This is pretty common.

As @ebutusov said, *PS project contains implementations for Proxy and Stub. They are not standard, instead they are generated by MIDL for interfaces exported from your ATL server. These interfaces are declared in the *.IDL file. The ouput of the project is DLL. You may read this article to get more details.

You may remove PS project from the solution in case if you do not define any custom interfaces in you *.IDL file or if you define only interfaces which have dual and oleautomation modifiers. In that case a standard typelib marshaller will be used.

In order to be able to make use of the standard typelib marshaller, one has to register a typelibrary (which is done automatically since you are using ATL)

It's proxy/stub code, which contains non-standard data marshallers needed to transfer data between different apartments (threading related). It's used when application, which calls your COM object, uses different COM threading model. There was an option in ATL/COM wizard to merge this code into main library. In many common scenarios you don't have to worry about it (i.e. when your COM dll runs in the client context), unless you want to write a custom marshaller.