The Fixed and Immovable restraints are identical for solid meshes. They are different for shell meshes. The following example illustrates the difference between the Immovable and Fixed restraints for shell meshes:
Consider a thin plate that is simply supported at two opposite ends. A uniform normal pressure is applied to the shell.
The following schematic figures illustrate the deformed shape (blue dashed curve) of the shell when the Immovable and Fixed restraints are used.
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Immovable: Finite rotations at the immovable edges (slope of the deflected shape is not zero).
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Fixed: No rotations at the fixed edges (slope of the deflected shape is zero).
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