Tags

Tags Precedence

Tags allow tagging all kinds of Matroska parts with very detailed metadata in multiple languages.

Some Matroska elements also contain their own string value, like the track Name element ((#name-element)) or the ChapString element ((#chapstring-element)).

The following Matroska elements can also be defined with tags:

• The track Name element ((#name-element)) corresponds to a tag with the TagTrackUID ((#tagtrackuid-element)) set to the given track, a TagName of TITLE ((#tagname-element)), and a TagLanguage ((#taglanguage-element)) or TagLanguageBCP47 ((#taglanguagebcp47-element)) of “und”.

• The ChapString element ((#chapstring-element)) corresponds to a tag with the TagChapterUID ((#tagchapteruid-element)) set to the same chapter UID, a TagName of TITLE ((#tagname-element)), and a TagLanguage ((#taglanguage-element)) or TagLanguageBCP47 ((#taglanguagebcp47-element)) matching the ChapLanguage ((#chaplanguage-element)) or ChapLanguageBCP47 ((#chaplanguagebcp47-element)), respectively.

• The FileDescription element ((#filedescription-element)) of an attachment corresponds to a tag with the TagAttachmentUID ((#tagattachmentuid-element)) set to the given attachment, a TagName of TITLE ((#tagname-element)), and a TagLanguage ((#taglanguage-element)) or TagLanguageBCP47 ((#taglanguagebcp47-element)) of “und”.

When both values exist in the file, the value found in Tags takes precedence over the value found in the original location of the element. For example, if you have a TrackEntry\Name element and a tag value TITLE for that track in a Matroska Segment, the tag value string SHOULD be used instead of the TrackEntry\Name string to identify the track.

As the Tag element is optional, a lot of Matroska Readers do not handle it and will not use the tags value when it’s found. Thus, for maximum compatibility, it’s usually better to put the strings in the TrackEntry, ChapterAtom, and Attachments elements and keep the tags matching these values if tags are also used.

Tag Levels

Tag elements allow tagging information on multiple levels, with each level having a TargetTypeValue ((#targettypevalue-element)). An element for a given TargetTypeValue also applies to the lower levels denoted by smaller TargetTypeValue values. If an upper value doesn’t apply to a level but the actual value to use is not known, an empty TagString ((#tagstring-element)) or an empty TagBinary ((#tagbinary-element)) MUST be used as the tag value for this level.

See [@?I-D.ietf-cellar-tags] for more details on common tag names, types, and descriptions.