The artist commentary fields are used to capture any additional information the artist has provided, namely the title and caption of the work.
Deciding whether to add commentary
Use your best judgment as to whether to include commentary or not. If the artist commentary is not in a language you understand, you should more often than not add it in so that other users may translate it when they see it.
Note that adding commentary does no harm, whether it is useful or not. Commentary can easily be removed if determined to not be of much worth. Leaving out commentary risks it being lost in the future. The commentaries of works with bad id or bad twitter id for example, if not included during the upload process, cannot be found after.
Although adding commentary is entirely optional, it is highly suggested.
Adding the artist's commentary
You can add or edit information in these fields by viewing a post and then clicking the Add artist commentary link in the sidebar. A dialog with four fields will appear. Original title and Original description should have the artist's original commentary, regardless of the language it's in. Commentary can automatically be fetched from the source link or another external link, or copied from another post altogether.
In addition to adding in information after a post has been uploaded, commentary can be added during the uploading process by checking the box next to Include Commentary.
If the artist posted multiple distinct commentaries across multiple sources, then preface each commentary with "Source Name:" on its own line. (e.g. post #2387146)
If the artist has also posted any interesting tags, these can be listed at the bottom of the commentary field (e.g. post #2519177).
Commentaries from multiple sources
Often, an artist will post a particular picture at multiple sources, using a different commentary for each source.
In those cases, use the h4 dtext tag for source headings, h5 for titles, and everything goes in the body. Additionally, use textile links on the sources pointing to the original source material for the comment (help:dtext).
See post #2387146 for an example of this in action.
Translating the artist's commentary
Translations of the commentary can be placed in the Translated title and Translated description fields in the Add artist commentary dialog.
Once all relevant parts of the commentary are translated into English, the commentary request tag should be removed and replaced with the commentary tag. If you are unsure of the translation, add the check commentary tag.
See help:dtext for help with formatting commentary. Note that DText does not work in commentary titles, only the description.