SAP2000 v10 - Differences between "plane" and "Shell"

Hmmm... you must find the analysis reference manual (Saprefer.pdf). There you can read:

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The Shell element is used to model shell, membrane, and plate behavior in planar and three-dimensional structures. The shell element/object is one type of area object. Depending on the type of section properties you assign to an area, the object
could also be used to model plane stress/strain and axisymmetric solid behavior.

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The Plane ele ment is used to model plane-stress and plane-strain behavior in two-dimensional solids. The Plane element/object is one type of area object. Depending on the type of section properties you assign to an area, the object could also be used to model shell and axisymmetric solid be havior.