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). These entities 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 inside the FE Mesh. These elements are beams, shells, or cells (including interior cells).
• Can be used to define the material and elemental properties of 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, and 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
• How to run a static analysis