The modelling paradigm provided by SAIF is quite independent of the low level (binary) encoding scheme used to carry data in typical operations. A number of possible options exist which may be used for this purpose, including SAIF/ZIP (a very efficient technique based on compressed ASCII), another built on the eXternal Data Representation (XDR) specification, and still others based on other specifications. Typically all data relevant to the description of an object or a set of objects are encoded in the data stream or file. However, it is also possible to provide references to data held externally, in the same or different formats. Data transfer formats are the subject of the companion volume, Spatial Archive and Interchange Format: Encoding Schemes.