Add subtitle->audio track linking

ebml_matroska.xml

@@ -508,6 +508,15 @@ The format depends on the `ChapProcessCodecID` used; see (#chapprocesscodecid-el
     <documentation lang="en" purpose="definition">Specifies a chapter edition UID to which this `TrackTranslate` applies.</documentation>
     <documentation lang="en" purpose="usage notes">When no `TrackTranslateEditionUID` is specified in the `TrackTranslate`, the `TrackTranslate` applies to all chapter editions found in the `Segment` using the given `TrackTranslateCodec`.</documentation>
   </element>
+  <element name="TrackLinks" path="\Segment\Tracks\TrackEntry\TrackLinks" id="0x6FAC" type="master" minver="5" maxOccurs="1">
+    <documentation lang="en" purpose="definition">List of associated audio tracks for a given subtitle track.</documentation>
+    <documentation lang="en" purpose="usage notes">The parent `TrackEntry` **MUST** have a TrackType of `subtitle`.</documentation>
+  </element>
+  <element name="TrackLink" path="\Segment\Tracks\TrackEntry\TrackLinks\TrackLink" id="0x6FAD" minver="5" minOccurs="1" type="uinteger">
+    <documentation lang="en" purpose="definition">Specify that a subtitle track is suitable for use with the audio Track UID specified (in the u-integer).</documentation>
+    <documentation lang="en" purpose="usage notes">The value **MUST** match the `UID` value of an audio track found in this `Segment`.
+Multiple subtitle tracks may link to the same audio track, and the same subtitle track may link to multiple audio tracks by providing multiple TrackLink elements.</documentation>
+  </element>
   <element name="Video" path="\Segment\Tracks\TrackEntry\Video" id="0xE0" type="master" maxOccurs="1">
     <documentation lang="en" purpose="definition">Video settings.</documentation>
     <extension type="libmatroska" cppname="TrackVideo"/>

notes.md

@@ -1093,6 +1093,27 @@ An overlay track **SHOULD** be rendered in the same channel as the track it's li
 When content is found in such a track, it **SHOULD** be played on the rendering channel
 instead of the original track.
 
+## Track Links
+
+A track link indicates that a given subtitle track is suitable for use with a given audio track,
+and that it **SHOULD** be eligible for autoselection ((#default-track-selection)) when that audio track is selected.
+
+For instance:
+- A subtitle track containing a translation of an audio track in another language should be
+  linked to that track, but should not be linked to another audio track featuring a dub
+  into the same language with a different translation.
+  A subtitle track containing a transcription of the dub, if present, should be linked to the dub audio track.
+- A forced subtitle track containing onscreen text consistent with the dialogue in an audio track
+  of the same language should be linked to that track, but should not be linked to another audio track
+  featuring a different translation into the same language.
+- A subtitle track containing a transcription of an audio commentary track should be linked to
+  that audio commentary track, but not to other tracks featuring separate commentary recordings.
+
+Track links are only necessary when the set of tracks available creates ambiguity about which
+subtitles are appropriate for which audio, and function as a hint to the player to improve
+its automatic track selection behavior. In cases where there is no such ambiguity, there is
+no need to explicitly specify track links.
+
 ## Multi-planar and 3D Videos
 
 There are two different ways to compress 3D videos: have each eye track in a separate track
@@ -1238,3 +1259,5 @@ the Forced track should be selected nonetheless, rather than selecting no track
 On the other hand, if the user's preferences _do_ call for captions, the non-Forced tracks
 should be preferred, as the Forced track will not contain captioning for the dialogue.
 
+If a given subtitle track has any TrackLink entries ((#track-links)), it should only be eligible
+for default selection if one of the linked audio tracks has been selected (whether automatically or explicitly).