An Appendix was first added to the second edition of The International Classification of Headache Disorders (ICHD-II). It had several purposes, which are retained in ICHD-3.

The primary purpose of the Appendix is to present research criteria for a number of novel entities that have not been sufficiently validated by research conducted so far. The experience of the experts in the Classification Committee, and publications of variable quality, suggest that there are still a number of diagnostic entitities that are believed to be real but for which better scientific evidence must be presented before they can be formally accepted. Therefore, as has happened between ICHD-II, ICHD-3 (beta) and ICHD-3, it is anticipated that some disorders now in the Appendix will move into the main body of the classification at the next revision.

In a few places the Appendix presents alternative sets of diagnostic criteria to those in the main body of the classification. This is again because clinical experience and a certain amount of published evidence suggest that the alternative criteria may be preferable, but the committee does not yet feel that the evidence is sufficient to change the main classification.

Finally, the Appendix is used as a first step in eliminating disorders historically included as diagnostic entities in previous editions of ICHD, but for which sufficient evidence has still not been published.