For danbooru purposes a circle cut (サークルカット) is a special type of advertisement for a doujin circle. There are two basic types pictures tagged as circle cut. The traditional variety has two distinct features:
- They are (almost) always monochrome;
- There is always a little, white, black bordered box in the upper left corner.
The box is there for specific information, such as when the doujin circle will be attending an event and where their stand exactly is. See this image: http://yaplog.jp/cv/raitika/img/78/0_p.jpg for an example of a filled out circle cut.
The second kind doesn't need to conform to the visual template exactly, but is characterised by having been released as a direct preparation of the exposition in question. They might vary in appearance from just a character's portrait + circle name, up to entire small scenes or comics whose purpose is to announce the booth number. For example, see post #634255 and post #161515.