56 Personal Content Experience: Managing Digital Life in the Mobile Age Even with this classifi cation, we should be aware that the categories are not unambiguous. Any piece of content may belong in several classes, such as an e-mail may be in the reading class at the same time as in a communication class. Nevertheless, there is a category for each piece of content. In addition to classifying content by internal representation or func- tion, there are several additional approaches. We will discuss some alternatives briefl y. Many of them are better suited for categorization needs in special circumstances, not for general classifi cation. Personal content types can be classifi ed by their maturity. Is the content type declining (such as voice mail); is it mainstream (such as short messages are at the time of writing); is it emerging (such as mul- timedia messaging); or is it merely a weak signal of a potential new content type in the future (such as context data is at the time of writing)? This kind of classifi cation may be useful in some cases, for example, when developing new personal content services that need to scale up for future needs. The content can be classifi ed by its creation. Is it created by the user; has it been created together with others; or has it been created totally by others? A need for this kind of classifi cation may emerge, for instance, when dealing with copyright issues. Related to creation is also classifi ca- tion by automation – has the content object been created explicitly by the user, such as a saved game, or implicitly, such as a heart rate log? The problem often related to implicitly created content is its second- class citizenship since no explicit effort has been put into the content creation as such. However, losing a log of all sporting activities for the last three years, for instance, could be considered catastrophic. At least some personal content can be defi ned by its expected lifes- pan. Is it targeted at one-time enjoyment, such as a rapidly spreading funny e-mail message on a Friday afternoon, or is it a piece of memory that should live forever? This aspect needs to be considered, for example, when designing management systems. Some content needs to be acces- sible instantly, whereas sometimes it is enough to know that a certain piece of content exists and it can be retrieved when needed. In a similar manner, the need for backups and archiving can be considered. The list of content classifi cation schemes given above is certainly not exhaustive. On the contrary, new schemes can and should be generated when needed. 3.3 Characteristics of Personal Content What does personal content look like? This section aims at pre - senting some of its typical characteristics. The purpose is not only to