XML Companion, The, Third Edition

Formal public identifiers

A formal definition for the format of public identifiers exists, and was originally defined for use with SGML. These identifiers include useful information about the resource, including the owner of the entity, the nature of its contents and even the human language used. A formal public identifier has a rigid structure composed of several parts: the identifier type, the owner identifier, the public text class, the public text description and the public text language. A public identifier should either conform strictly to the rules described below, or ignore this standard completely. Adopting the appearance but not the rigid rules of a formal public identifier would be confusing to people attempting to interpret it.