mkvalidator warning WRN201 "unknown ... for read profile 'matroska v2'"
When running mkvalidator 0.4.2 on a file generated by mkvmerge 5.9.0 or later it issues the following warnings:
WRN201: Unknown 'CueRelativePosition' for read profile 'matroska v2' in CueTrackPositions at 5992213076WRN201: Unknown 'CueDuration' for read profile 'matroska v2' in CueTrackPositions at 5992213076
What do they mean?
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, but it does not have to be equal to it.
The two elements that mkvalidator complains about are elements that have been added in Matroska v4 in order to make seeking easier and more precise for players. They're not strictly required for playback, nor for seeking, which is why a player not supporting v4 but the earlier version v2 should not have any problem playing such files. That is why mkvmerge sets DocTypeVersion to 4 (as it writes elements from version 4 of the specs) and DocTypeReadVersion to 2 (as a player supporting v2 but not yet v4 can still play the file just fine).
This situation is normal and in accordance with both the letter and the intent of the specs. mkvalidator agrees; otherwise this would be an error.
As to why it is a warning and what the harm should be: no idea; please ask Steve Lhomme, the author of mkvalidator.