Any two topics that share one or more subject indicators (or that have the same subject address, in the case of addressable subjects) are considered to be semantically equivalent to a single topic that has the union of the characteristics (the names, occurrences and associations) of both topics. In a processed topic map a single topic node results from combining the characteristics of the two topics.[7] A subject indicator could be an official, publicly available document (for example, the ISO standard that defines 2- and 3-letter country codes), or it could simply be a definitional description within (or outside) one of the topic maps. A published subject indicator (PSI; originally called “public subject descriptor”) is a subject indicator that is published and maintained at an advertised address for the purpose of facilitating knowledge interchange and mergeability, either through topic maps or by other means. Published subjects are a necessary precondition for the widespread use of portable topic maps, since there is no point in offering a topic map to others if it is not guaranteed to “match up” with relevant occurrences in the receiver's pool of information resources.