A tetrahedron that has all four nodes on a surface, distorts calculations in that area. This is called insufficient refinement through the thickness.
A 3D model with insufficient refinement through the thickness is detected with the Mesh Repair Wizard: 
().
Description
There are two cases where insufficient layers occurs:
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Nodes on opposite surfaces being directly connected
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Tetrahedra (tetra) with all four nodes on a surface
When constructing a 3D mesh, it is important that a node on one surface does not connect to a node on the opposite surface (AE below). There should be at least one intermediate node between surface nodes (e.g. BF, CG, DH. etc.)
Node arrangement
If a 3D mesh has been imported from an external CAD package or manually edited, it is possible that a tetra could have the nodes AEFX in the figure below. This element has four nodes on the surface. The Flow solver assumes that the flow velocity at a surface node is zero. An element with all nodes on a surfaces will then never be filled. Current models overcome this by force filling these areas. This distorts all the calculations in the area for flow, temperature etc.
Unrefined tetra
Impact on the analysis
Insufficient mesh refinement through the thickness can reduce the accuracy of results that typically have significant variation across the thickness. In particular the accuracy of thermal, velocity, shear rate and fibre orientation results can be impacted. Conversely very fine mesh can lead to a significant increase in computational overheads.
Recommended action
It is important that regions identified as having insufficient layers are assessed and refined, particularly in regions where detailed thermal, velocity, shear rate and fibre orientation results are required.
To refine the mesh, go to 
(), and enter the minimum number of elements through the thickness required in the text box under the Tetra tab. Ensure Remesh already meshed parts of the model is selected and then click Mesh Now.