This bag is a small trunk I bought on ebay, with a shoulder strap which clips on as a try out for d.i.y. It can easily carry stuff like camera, phone, sandwiches, small tools, bicycle pump etc.
The adapter plate is simply bolted on through the bottom of the bag. You can see the silver nuts. The four pointed parts simply go through the Haibike Carrymore rack cross members and get pushed forward until the front catch with the blue button engages in some notches. The bag is then held firmly in place while riding.
To remove, the blue button gets pushed and bag is free in seconds. Very convenient and no messing with Velcro straps.
Panniers and other sizes of bag could easily be attached to adapter plate too and thus be a convenient quick and easy attachment