This tag is used to indicate when it is okay to draw Fanart of an Original character or object. The artist who created the original design posts this tag to tell other artists that they fully encourage anyone else to draw their creation. However, one should not be confused with reposting the original artist's work elsewhere outside pixiv without permission
As is common courtesy, be sure to link back to the creator's illustration or pixiv profile when you post your own drawing. You can also use the image response feature to link your illustration to the original artist's illustration.
It may still be acceptable to create fanart for images without this tag, but be sure to ask the artist first via private message. Some illustrations may not be suitable for fanart and some artists may not wish for their creations to be drawn by other artists. Please respect the artist's wishes if this is the case.
Commonly illustrations related to various "group events" on Pixiv are assumed to have this tag even if it is not listed. For example Pixiv Fantasia is an event where many artists create characters and comics featuring each others creations. In that instance few if any of the images will have this tag, but virtually all of the characters are open to be drawn by other participating artists. Be sure to check the event announcement to be sure.