GLOSSARY
Smoothing groups define whether a surface is rendered with sharp edges or smooth surfaces.
Smoothing groups are numbers assigned to the faces of an object. Each face can carry any number of smoothing groups up to the maximum of 32. If two faces share an edge and share the same smoothing group, they will render as a smooth surface. If they don’t share the same smoothing group, the edge between them will render as a corner.
You can manually change or animate smoothing group assignment.