An appendix holds the detailed supporting material that would clutter your main chapters. Include enough data and explanation that curious readers can find answers to their questions without bogging down your core arguments.
How detailed does my appendix really need to be? Your appendix is where you put things that support the material. Usually appendices are pretty detailed because it's supporting material as opposed to putting all the fine detail in the chapter 4 or whatever it's going to be. It's enough so that if the reader is curious, they can go there and get their answers.
