What is the Tag field for?

[Trip Machine]

Say in VDubMod when I add in an audio stream, it brings up a menu where I can edit the properties and information of the stream.

There is a drop-down menu in which you can select options like Language, Description, Artist, Copyright, etc. To the right of the drop-down menu, there is a Tag and Value field.

As the post implies, what's the Tag field used for? If I select my Language to be English, Value becomes English[eng/en]. I was reading the Doom9 VDubMod setup procedure guide and he made no mention of it or what it does.

[lamer_de]

It's only usefull for ogm files (afaik) When you have more than one audio stream, you can read the thing you entered there instead of stream 1, stream 2 or similar. It's only cosmetic/info stuff, nothing to worry about if you don't want to tag your avis/ogms

CU,
lamer_de

[Suiryc]

The purpose of those fields is to let the user add a wide range of comments in the output file.

As you already noticed and mentionned, you can select a kind of comment thanks to a dropdown list, and there are 2 fields named Tag and Value.
What you can/cannot do depends on the type of comment you are dealing with.
In the case of specific/general comments, you fill yourself the Tag and Value fields.
For example, with an OGM file you could use a "TITLE" Tag and a "That's my title" Value to set the title of your file, and for an AVI file you would have to use the "INAM" - IIRC - Tag to set the title of your file.
As you can see this is quite annoying to have to know that you have to use "TITLE" for OGM and "INAM" for AVI when you want to set the title of your file. That's where the "Type" drop-down list is useful. You select a kind of comment (in our case a "Title" comment) and that's up to the program to do the necessary when writing the output file (i.e. use "TITLE" for OGM and "INAM" for AVI).
Of course in this case the Tag field become useless (and thus is not editable). You may then ask why I still write (in the list of comments) what's the current 'Tag' value for such comments. This is to let you know what was the original (or previous) value of this field (in case you want to make a specific/general comment).

Language comments are a bit different. When selecting a user defined Language, as for other cases you can fill yourself the Value field, but not the Tag one. However when you select a defined Language then you can't edit th Value field (what you show in this field just tell you what language you selected, it's up to the program to determine what to do with this Language).