162 Personal Content Experience: Managing Digital Life in the Mobile Age http://namespaces.example.com/metadata/game-metadata/ that would refer to example.com’s game-related metadata defi nitions. 5.6.5 Metadata Schema Objects This ontology defi nes fi ve main schema classes that contain all attri- butes. All instance objects are expected to include at least either the Object schema or the Content Object schema. The Object schema contains the least amount of metadata any object in the system must have. Therefore, the content types of all instances having only the Object schema are basically unknown. The Content Object schema object adds a few more properties that are applicable to most objects. Using the Content Object schema is strongly recommended as a minimum set of metadata attributes for any object. In addition to this minimum set of attributes, all instance objects are expected to take one of the more specifi c set of attributes. For example, all jpg images include the Image schema. Similarly, music content such as MP3s and AAC fi les need to include the Audio schema and the Time- based schema in addition to the Content Object schema. Videos are an interesting case. They certainly need all Time-based attributes and all Image attributes added to Content Object attributes, but they also may need all audio-related attributes for the audio track. This ontology has an empty schema object for common media formats. They may contain format-specifi c metadata attributes but their main reason is to make searching easier. All schema objects can be used as fi lters, implying that only the instance objects using the schema are selected. For example, with this schema, you can search for all audio objects including either MP3 and aac fi les, or just MP3 fi les. The tables below contain all defi ned metadata attributes and schema objects. Attribute column gives the name of the attribute, type columns its data type, N column is the cardinality (how many time the attribute can be given to a single metadata instance object), and fi nally a comment giving a brief summary of its purpose. The comment is not intended as a fi nal document for the attribute but is given here only as an example. There must always be a separate document describing the ontology in detail. A number in the N column means that the attribute must exist exactly as many times as the number given. For example, number one for creationDate attribute says that creationDate must always exist and there must be only one of them in any instance. Numbers may also have a “+” sign, which implies that the attribute can be given multiple