Where the cutting plane intersects the shell cleanly at 90 degrees to its plane, the well-conditioned case, the cut plane through the element gives a good representation of the true element thickness.
However when the plane cuts the plane at an oblique angle as shown in the images below the calculation of the cut place depends on the "2D and 1D section cut"setting in the Options panel.
This has two possible settings
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T90 degree cut , in which the cut surface calculated is always that of the well-conditioned orthogonal cut, which is the default because it is a "safe" option when computing section properties.
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True cut , in which the actual surface generated by the intersection with an oblique plane is computed. This is useful for visualisation of actual geometry, but will tend to over-estimate section properties.
These two options are illustrated below: | 
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90 degree cut (default)
In this case the effective cut plane through the element is always orthogonal, ie at 90 degrees to its plane, regardless of the actual angle at which the section cuts the element.
This is a "safe" method when calculating section properties, since otherwise a very oblique cut through a shell can result in a an area much deeper that its actual thickness which, in turn, could lead to the section properties being over-estimated.
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