What can be purpose behind forward declared class without defining and using it anywhere in the project in C++?

Question

I was exploring famous OBS Studio source code. That is using one git sub-module at
https://github.com/obsproject/libdshowcapture

In the file source/capture-filter.hpp there is a forward declaration of a class which is nowhere defined and used in whole repo and whole OBS-Studio repo.

namespace DShow {

// some other declaration

// this class is nowhere defined and used
class CaptureSource;

//some other declaration and definitions
}

I checked carefully but did not find any definition nor usage of CaptureSource class. If I am right then what can be purpose of such declaration?