Let me start by describing what is desired and then we will see if anyone can answer this question.
Image you have a part model with hundreds of surfaces. Now you would like to define an enclosed boundary from a combination of the surface edges. Then select all surfaces either inside or outside of the boundary. I do not believe this is possible to do with any of the core tools in SolidWorks but I would love to be proven wrong on that. Does anyone know if there is a macro out there that someone created that does this. I am contemplating creating a macro but thought I would throw this out there so as not to reinvent the wheel.