Overview
This tag is usable only if you happen to receive work requests or official, paid commissions from media companies. These works may be used for, but not limited to : Light novels, game characters, settings, art exhibition, promotion and many more. Usually these works are for commercial use, used by the artist to mainly promote their own products (light novel, game releases, etc) to their followers weeks or months before the release. However, this tag can also be used for doujins as well.
Please note that these artists do not draw this for the sake of fun. These works are actually requested from the companies that commissioned them. When such artwork is posted in pixiv, it means the both the company and the artist have agreed and allowed the particular work to be posted on the site.
Without the permission from the artist AND the company beforehand, one should NEVER use the artwork for other purposes (especially reposting it on other websites). As the artwork belongs to both the artist and the company, both sides own the rights for it.
Regardless, it is considered a violation the moment you use the artwork outside pixiv without permission, so be sure to think twice before reposting.