The appendix is the repository for those items that aren't part of the plan itself but that are helpful or useful to someone reading the plan. It contains the material that supports and explains the conclusions and assumptions contained within the plan. If you think it is likely that a reader will seek further information regarding some portion of the plan, include the appropriate material in the appendix. That way, you avoid having to dig through the material you accumulated while preparing the plan if a question arises as to the information presented within the body of the plan.
The appendix also houses, for example, sample marketing material. If you plan to run magazine ads, copies of the ads could be included in the appendix. Consider material for inclusion only if you believe that it adds to or clarifies the rest of the plan. If you are just starting out, consider including resumes of key employees if you are relying on their skill and experience.