BSO, also termed SRC (Subject-field Reference Code), is a classification system developed within the UNISIST-program for the purpose of interconnection of information systems. It is a disciplinary organized system founded in 1972 as a UNESCO project in the UNISIST-program "World Science Information System" in cooperation with FID. The idea behind BSO is related to the idea of networks and probably represents the latest attempt to create a new universal classification. It is developed by the Englishman Eric Coates in cooperation with others, including the Bliss Classification Association.
BSO was meant to be an international switching language, an overall information retrieval language to transfer blocks of information in coarse subject groups between information systems applying different indexing languages.
Coates (1979) states: "theoretically the switching operation requires nothing more than a neutral code system in which concepts are represented". Dahlberg (1978) regards it as a positive step towards standardization: "A standard classification assists library rationalization and national and international cooperation on statistics, research and cataloguing".
As referred in the entry on switching language, has critical voices been raised towards the theoretical assumptions behind such systems. BSO did not develop to fulfill the wishes behind its construction. This may have a connection to such inherent problematic assumptions in the concept of switching languages.