Hello there 
this is either a very silly question or indeed an interesting request…in the past i asked if anyone knew of a simple and effective way to select or mask or hide or show faces by their orientation (x,y,z) important for everything from things like shading or selecting faces for Uv unwraping and many other things…even better will be constraints based on first selected normal (using the picker) for example?
I know we have cavity masking or smoothness masking but this is a more specific way to select things
a simple example would be to say put a snow shader everywhere starting at Y = 200 meters or a proprtion equivalent in teh mesh…anotehr will be to select faces only facing camera (i had a limited sucess using back face masking) etc
if anyone one has maybe a good combo that does just this i will appreciate…Maya has a number of selection constraints that are quite esoteric at times and also forget about doing this of course on heavy meshes…
cheers