"An Agreement Addendum is, of course, needed to implement and add further meaning to these clauses. In that regard, it appears that JBI and RockTenn reached an agreement as to the basic form of any Agreement Addendum executed in accordance with the Master Agreement, given that it was incorporated by reference into the contract."
1.1.3. “Agreement Addendum” shall mean an addendum that may be from time to time executed by the Parties that is in substantially the form as attached hereto as Exhibit A."
They referenced it, but unfortunately chose not to attach it, which one would expect them to do as an exhibit to a material agreement. There is no indication that confidentiality was sought for it. In its absence we don't know whether Exhibit A is a fill-in-the-blanks guide as to form or a completed Agreement Addendum put forth as an example for future agreement addenda.
Or: You can leave your stuff at my house and come over anytime to use it and I promise not to let anyone else use your stuff, as long as we agree in advance what room you're going to leave it in. We'll talk.