org.cell2d.celickand its subpackages consist of classes directly copied, with or without modifications, from Slick2D, the library on which Cell2D is based. They are sometimes buggy, often poorly documented, and thus largely hidden away. Cell2D's online Javadoc excludes them, and the most you'll ever need to interact with them is passing instances of the Graphics class around as method parameters. You might have some use for them, though, in particular Graphics' functionality of drawing basic shapes on the screen. If you do, the javadoc in the Cell2D download includes them, and you can find that same javadoc online here - but tread with caution.