Why do some Boundary Conditions use an FE Region instead of an FE Boundary?

Question:

What's the difference between an FE Boundary and an FE Region? 

Why do some Boundary Conditions use an FE Region instead of an FE Boundary?

Applies to:

• Simulation
• Topology Optimization

Answer:

The FE Boundary and FE Region are used for choosing different elements on the mesh. 

FE Boundary:

• Can only select boundary entities (sub-elements) that are nodes, faces, or edges
• These entities are only found on the outside of the mesh

FE Region:

• Can only select elements that are present in the FE Mesh that are beams, shells, or cells (including interior cells)
• Can be used to define material and elemental properties for the selected elements

Most of the boundary conditions take only boundary entities while some (like heat generation) need actual FE elements (region) entities like cells, shells, beams.

More on this topic:

• How to Select Boundaries of an FE Mesh - FE Boundary by Body
• How to Select Boundaries of an FE Mesh - FE Boundary by Flood Fill