What's the purpose of `CXCallingConv_Default`?

I’ve been playing a bit with libclang and function declarations with different calling conventions and I’ve noticed that the cursor enum CXCallingConv has this Default variant but CallingConv does not, I’m wondering what’s its purpose and how does it interact with attributes like cdecl and the like. As far as I’ve noticed, this Default is never used for anything and C is used as the default calling convention everywhere.