Original 100 Users Bookmark is one of the ranking tags in pixiv.
The tag is used for illustrations with more than 100 bookmarks received. However, there are few things to note :
1. This tag will be used only and exclusively for original illustrations.
2. Fanarts or derivative works that did not have its special category will be labelled using the "100 Users Bookmark" tag instead. Unless the show is moderately/highly popular, then a particular user bookmarks category will be created for it instead. For example : VOCALOID 100 Users Bookmark
3. Considering this tag is used only for highly ranked original illustrations, to receiving this tag requires an amount of drawing skills, passion, creativity, patience, and luck.
4. One should NEVER use this to tag their own artworks, this tag will definitely be used when 100 bookmarks for an artwork are reached.