2.1.10 [ISO32000-1] Section 8.7.4.5.6, Type 5 Shadings (Lattice-Form Gouraud-Shaded Triangle Meshes)
V0010:
The specification states:
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Type 5 shadings (lattice-form Gouraud-shaded triangle meshes) are similar to type 4, but instead of using free-form geometry, their vertices are arranged in a pseudorectangular lattice, which is topologically equivalent to a rectangular grid.
Microsoft Edge:
Lattice-form Gouraud-shaded triangle mesh shadings are not supported (their presence is ignored).