Any overall explanation of design not covered by the previous two items. Given the scenario of a user making a shot, show the communication paths between the classes given in the design. Use a UML sequence diagram for this. Description of the proposed breakdown of work between the two partners. The milestone document may be handwritten.
Content is more important than format.
However, the more work you have put in for this milestone, the smoother the rest of the project will go. E.g., if you have fleshed out more details about the exact interfaces between your classes, and the representations they will use, include that info in your milestone document. Getting it checked off in person is a good idea because you'll get immediate feedback on possible improvements to your design.