What's a BlueprintGeneratedClass

Hi,
I need some help to understand BlueprintGeneratedClass better.

I made a new class based on the LandscapeBlueprintBrush and I need to add a new variable of type LandmassBrushManager.

It’s easy to do it with blueprints, but I couldn’t find how I could add it with CPP.

When I check the type of the object, it says it’s a BlueprintGeneratedClass.

Anyone know how I can add this variable with CPP?

I tried the C++ Header Preview tool and it seems I have to define it like this.

But if this is the solution, what should I do in the next step to implement it correctly?