This measure applies to quad, prism and hexa elements (for quadratic elements the linear part will be checked).
To determine the warp of a quadrilateral face, the angles between the triangles connected at the 2 diagonals of the quad will be calculated and the maximum will be used.
The maximum warp of a volume element (prism or hexa) is the maximum warp of its quad faces.
For a 2D planar mesh of QUAD_4 elements, the warp will be zero for all elements.