The addenda provide background information on some subjects of the site. The subjects are mentioned on several pages of the site so it makes sense to write a separate page on the subject instead of repeating the text again and again all over the site. The webpages refer to an addendum with an appropriate link. An example is the © sign and date in the top right corner of the pages. The link refers to Copyright Information. Addendum pages offer links to take you back to the page you came from. It makes no sense to study the addenda beforehand. You will see the links when you get there and I suggest you click only if you want to read more on the subject.