Short adapter shank #77 is the shortened version of Standard adapter shank #75. Serving as a connecting piece between needle bar and accessory, Short adapter shank #77 allows other manufacturers’ presser feet to be used on BERNINA machines.
If these feet are short-shanked, they won’t fit on the machine’s standard holder without an adapter. By contrast, Standard adapter shank #75 is used e.g. with BERNINA sewing accessories such as Simple binder #85 or Ruffler attachment #86.