A technical solution for easily carrying out detailed image formation control corresponding to respective customers, i.e., a detachable unit, an image forming apparatus, a server computer, a computer program and client system for easily carrying out detailed image formation control corresponding to respective customers, is to be provided. The detachable unit is a detachable unit detachable to an image forming apparatus, the detachable unit has a memory part storing information containing control information, and upon mounting the detachable unit on an image forming apparatus main member, operation of the image forming apparatus is controlled based on the information stored in the memory part.