It is called Volkssport Associates. There are a lot of organizations that provide walks. The American Volkssport Association combines the opportunity to walk with an award system. As you found out when you first registered, you could participate, or walk for credit, or for credit and the award. When you turn in your start card, you will also turn in your green, blue, or purple books, which will then be stamped and returned to you.
Some people have both and some people have only one.
It is a personal decision whether to have an event, a distance, or both books. Write your name and address on it so if you lose it, it can be returned to you. If you paid for the event and award you can get an IVV credit stamp in your new book and receive your award for the walk you just finished. From now on, when you go to a volksmarch, you can register for credit and get your books stamped. When you have completed your 10th event, photocopy your book, and send the book to the American Volkssport Association at the address given on the front of the book.