As Matroska has evolved since 2002, many parts that were considered for use in the format were never used and often incorrectly designed. Many of the elements that were defined then are not found in any known files but were part of public specs. DivX also had a few custom elements that were designed for custom features.
In this appendix, we list elements that have a known ID that SHOULD NOT be reused to avoid colliding with existing files. These might be reassigned by IANA in the future if there are no more IDs for a given size. A short description of what each ID was used for is included, but the text is not normative.