CPS UNIT MANAGEMENT IN THE DISC FOR DOWNLOADED DATA CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority of copending U.S. Provisional Patent Application No. 61/041,126, filed March 31, 2008, entitled "CPS Unit Management in the Disc for Downloaded Data." English-language translation of the above-referenced provisional application and statement that the translation is accurate has been filed in the provisional application. The disclosures of the above-referenced provisional application and the translation of the provisional application are incorporated herein by reference. BACKGROUND Field of the Invention The present invention relates to information processing. More specifically, the present invention relates to efficiently processing and acquiring subseguential data such that a content certificate updating process can be omitted. Background Audio data such as music, video data such as movies, game programs, various types of application programs, and the like, and various types of software data (hereinafter termed "content") can be stored on information recording media such as, for example, a Blu-ray Disc™ or a Digital Versatile Disc (DVD). The Blu-ray Disc™, which uses a blue laser, is a high-density recordable disc that is able to record high-resolution data such as large volume movie content. For many types of content, such as music data or video data, typically the distribution rights, or the like, are owned by the creator or the vendor. Consequently, when this type of content is provided to the user, there is typically a specific limit to its use. That is, the content is put into a form wherein use of the content is allowed only for authorized users. Specifically, control (e.g., an encoding process) is performed to prevent unauthorized reproduction. For example, the Blu-ray Disc™ has a standard wherein the content stored on the disc is divided into content control units (CPS units), and control is performed by each unit by corresponding usage control information ("usage rules"). Usage of content belonging to a single CPS unit is allowed within the scope established by the usage control information (usage rule) associated with the CPS unit to which that content belongs. The usage control information (usage rule) is recorded on the disc along with the content. When content is read, the reading device reads out the usage control information associated with the CPS unit to which the read content belongs, and the content is used in accordance with the usage control information. The usage control information (usage rule) includes, for example, copy control information (CCI) for the content, and has information such as whether or not the content may be copied, and modes wherein use is or is not allowed. When the content belonging to the CPS unit is encrypted content, then an association is made with a CPS unit key, which is an encryption key corresponding to the unit. When reading the encrypted content, a decoding process is performed using the CPS unit key. Note that a CPS unit key file, where the CPS unit key is stored, is also recorded on the disc. Content that is stored on a Blu-ray Disc™ is used and controlled under this type of strict usage control. Moreover, the Blu-ray Disc™ standard also regulates the use and control of data, as subsequential data, when downloading, from the server, adjunct data corresponding to the content recorded on the disc, such as subtitles or alternate soundtrack data, or other service data or video or still image content, or the like corresponding to the content. Systems for using content wherein subsequential data is obtained from a server, or the like, to be recorded in a local storage unit, such as a hard disk, to be read along with the content that is stored on the disc are disclosed in, for example, Patent Reference 1 (Japanese Patent 3959735), and Patent Reference 2 (Japanese Unexamined Patent Application Publication 2003-140662). In the Blu-ray Disc™ standard it is a requirement that content information be recorded in a content certificate