Do I use stencils, MaskPass, or some other way to target specific objects with EffectComposer? How?

Question

I'm want to apply post-processing with EffectComposer to specific objects in my scene. So far I'm only able to apply effects such as FilmPass to the entire canvas as one big rectangle.

I'm not sure to begin with this. Do I pass paramters such as these:

{ minFilter: THREE.LinearFilter, magFilter: THREE.LinearFilter, format: THREE.RGBAFormat, stencilBuffer: true }

to my WebGLRenderTarget? Why does the stencil parameter only take a bool value instead of the mesh you want to use as the stencil? Should I be inserting a MaskPass into my render chain somewhere? In either approach, how do I indicate which mesh I want to target as the clipping path for my effects?