This tag is being populated on and off, so might as well get this implication out of the way (for the topic on the introduction of language commentaries tags, see: topic #14807)
The wiki for commentary says:
"Posts with this tag have the artist's accompanying commentary from the original site posted (and, if necessary, translated into English)".
English commentaries fall under the "commentary" tag because they don't need translation, as danbooru is an english speaking site.
Updated by DanbooruBot