The Oracle database consists of a number of physical files, often referred to as operating system files, since they are usually created and maintained by the host operating system. These physical files are used by Oracle for parameter storage, database coordination, and data storage. The files shown in the following list, which are described in the following sections, are unique to a particular database; that is, each mounted and opened database must have all of these files, and the files are not shared across instances except in the special case of the Oracle Parallel Server, whose purpose is to share datafiles.