168 Personal Content Experience: Managing Digital Life in the Mobile Age xml media types. So it is natural that XPointers are used whenever there is need to give metadata to parts of XML document. Note that the framework does not evaluate whether the pointer is valid. Evalua- tion is left to the application. 5.6.10.2 Time-based Content Objects Time-based content objects have duration. Consequently, a point in the object can be simply and uniquely defi ned by giving a point in time. Typically, video clips and audio streams are time-based content objects. For time-based fragments, we specify that the start and end times defi ne the fragment. The start time is used inclusively but end time exclusively, i.e., start time is considered as part of the fragment but end time is not. This way, it is possible to create non-overlapping time- based fragments. However, overlapping fragments can be created if needed. As with XPointers, the framework does not evaluate or be affected by how applications use fragments. Either the start or the end defi nition can be omitted, but not both. If the start time is omitted, 00:00:00 is assumed and if the end defi ni- tion is omitted (or if the end defi nition extends beyond the end of the playback), the end of the playback is assumed. Time fragment specifi cation is in the form #time(start=hh:mm:ss[:fraction],end=hh:mm:ss[:fraction]). The starting and ending times are expressed by hours, minutes, seconds, and an optional fractional part, indicating how much time has elapsed since the beginning of the playback. The fraction part depends on the type of the media. For video, it is the number of frames from the beginning of the second, and for audio it is milliseconds. For example, a fragment URI http://path/to/video.mpg#time(start=01:20:00, end=01:22:00) will point to a two-minute video clip that starts at 01:20:00 and ends at 02:22:00. 5.6.10.3 Coordinate Based Content Objects Coordinate-based content objects are media objects where fragments can be defi ned with geometrical shapes that have an area or a volume. Typical coordinate based objects are image fi les.