Thus, the print and electronic media versions of a serial need separate ISSNs, and CD-ROM versions and web versions require different ISSNs. However, the same ISSN can be used for different file formats (e.g. PDF and HTML) of the same online serial. This "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, and the Web, it makes sense to consider only "content", independent of media. This "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier (DOI), an ISSN-independent initiative, consolidated in the 2000s. Only later, in 2007, ISSN-L was defined in the new ISSN standard (ISO 3297:2007) as an "ISSN designated by the ISSN Network to enable collocation or versions of a continuing resource linking among the different media". An ISSN can be encoded as a uniform resource name (URN) by prefixing it with "". For example, "Rail" could be referred to as "". URN namespaces are case-sensitive, and the ISSN namespace is all caps. If the checksum digit is "X" then it is always encoded in uppercase in a URN. The URNs are content-oriented, but ISSN is media-oriented:   A unique URN for serials simplifies the search, recovery and delivery of data for various services including, in particular, search systems and knowledge databases. ISSN-L (see Linking ISSN above) was created to fill this gap. The two standard categories of media in which serials are most available are "print" and "electronic". In metadata contexts (e.g., JATS), these may have standard labels.  "p-ISSN" is a standard label for "Print ISSN", the ISSN for the print media (paper) version of a serial. Usually it is the "default media" and so the "default ISSN".  "e-ISSN" (or "eISSN") is a standard label for "Electronic ISSN", the ISSN for the electronic media (online) version of a serial.