In dental CAD/CAM manufacturing, an undercut is a surface region that the milling tool cannot reach from the available machining directions, or that collides with the path of insertion when the restoration is seated. Undercuts leave surfaces unmachined or prevent the restoration from seating on the preparation.
Why is an undercut not a mesh error?
The triangle mesh can be fully valid. An undercut only exists relative to a direction, the insertion axis or the tool orientation, which makes it a manufacturing question, not a question of file integrity.
Learn more: Undercut check in Mill Check