Membrane: used for conventional slab with beam.

Shell: is used for flat slab and wall. 

Why do beams located under membrane area objects generate higher moments than when under shell area objects?

Answer: Load which is applied to membrane objects transfers directly to supporting structural objects, whereas meshed shell objects have bending stiffness and therefore resist a portion of the load through flexural deformation. As a result, less load will be available to transfer to beams located under a shell, while 100% of the load will transfer through a membrane.