The BERNINA Overlocker Binder Accessories Holder (Attachment Base) is REQUIRED when using Binder Attachments #C21 and #C22 to attach to the machine. One Accessories Holder is interchangeable with the two Binder Attachments; the design makes it adaptable for use with multiple machines because it adjusts forward and backward as well as side to side. There are two pairs of screws (4 total). The shorter screws are for use with the L 890 and the longer pair will work with a variety of other coverstitch machines. The Accessories Holder will be compatible with future accessories to use on the BERNINA overlockers, allowing for many more possibilities and added value. Binder attachments are made for use with BERNINA L 890 machine and compatible other models. To see them and the accessories holder/attachment base in action, watch this how-to video: https://youtu.be/twINlpCW1UU.