A groove at the end of the thread that allows the mating part to sit flush against a shoulder.
The standard (normal) design for external thread undercuts. It is typically used for general engineering applications. din 76-a pdf
It creates a groove at the end of a thread that is smaller than the minor diameter of the bolt. This allows a mating part (like a nut) to screw all the way to the shoulder of the bolt without interference from the incomplete threads that occur at the end of a threading tool's path. Dimensions: For Type A, the maximum width of the undercut ( ) is typically is the thread pitch). A groove at the end of the thread
Designed for use with ISO metric coarse and fine threads as per DIN 13-1. Where to find the full standard: It creates a groove at the end of