mkvalidator error ERR009 read DocType version higher than DocType version
When running mkvalidator on a file generated by mkvmerge 5.9.0 or later it issues the following warning:
ERR009: The read DocType version 2 is higher than the Doctype version 4
What does this mean?
This is a bug in mkvalidator in versions up to and including 0.4.2. It has been fixed in v0.5.0. If you're running an earlier version please upgrade to the latest one.
The in-depth explanation
In Matroska there are two header fields that include information about the Matroska standards/specs version number used. One is the DocTypeVersion and one the DocTypeReadVersion.
The first one, the DocTypeVersion, must be set to the highest version number of the Matroska specs from which the muxer creating the file has used elements from. E.g. if the muxer used SimpleBlock elements then the DocTypeVersion must be at least 2 because the SimpleBlock element was added in Matroska v2.
The other element, DocTypeReadVersion, must be set to the minimum Matroska version that a player must support properly in order to be able to play back the file. This is meant to ensure that players that support older versions can still play files created by newer tools even if elements from newer versions of the specs are used.
Logically it is easy to see that DocTypeVersion must always be the same or higher than DocTypeReadVersion.
Starting with 5.9.0 mkvmerge writes two elements from the v4 specs, CueDuration and CueRelativePosition. Therefore mkvmerge sets DocTypeVersion to 4. However, those elements only provide additional information that is not strictly required for playback, not even for seeking (which they aim to improve). Therefore it leaves DocTypeReadVersion at 2.
And here's where the bug in mkvalidator lay: it compared DocTypeReadVersion < DocTypeVersion instead of the other way around. The error it therefore issued, however, is correct in its wording; only the numbers listed didn't make sense because they're taken directly from the file.