146 Personal Content Experience: Managing Digital Life in the Mobile Age of such a relationship, though, is evident only to its creator unless he or she publishes documentation about its semantics and thus leading to its wider adoption. The advantage of this approach is that Eddie’s loot may have its own metadata like updated date, passwords for access control, and so forth. Even if there is no physical fi le or object in the system as such, it still can have its own metadata. The main use of such relationships is for creating virtual objects. These objects exist purely in our frame- work, have metadata, and can play a part in relationships but they contain no fi les or bits. Thus, virtual objects are pure metadata. As can be seen, anything can have attached metadata, and in the case of virtual objects, even nothing can have metadata. Simply put, relationships are two-way links between content objects. They show that two or more objects have a common character. It does not matter what kind of objects they are, or whether there is a single application that can handle both types. As an example, you may want to add a link from a calendar event to a note describing the meeting, a link to the phonebook for participants’ phone numbers, and a link to an on-line game to be played at the event. The framework allows linking anything to everything and whether it makes sense or not is completely up to the creator of the link. The framework happily accepts and stores it for further reference. To achieve application interoperability and common metadata usage, we do have defi ned semantics for some of the most common relationships. If a developer decides to use those well-defi ned relation- ships, it is assumed that they follow these defi nitions. However, the framework has little means by which to enforce doing so. It is totally possible but probably disastrous to use, for example, the replaces rela- tionship to link two or more holiday trip photos to each other. However, some application that does automatic housekeeping or version control- ling might use that relationship as an indication that it is allowed to remove other images if storage space is limited. As we discussed earlier, automatic maintaining of backups and synchronization is especially important in mobile devices where the storage space is always limited. We have defi ned semantics of relationships just for ensuring interoper- ability between applications so they do know what the intentions of other applications are. One of the most useful relationships is the “is used by” relationship, which conveys that a content object is used by another one for some purpose. The relationship itself does not convey how the other object is used. That is left to the application developer but since all relation- ships are automatically two-way relationships, the used object also has metadata that it is being used. A practical example could be using