An essentially arbitrary string preceded by a question mark and delimited by angle brackets that is intended to convey information to an application that processes an SGML instance. The pitarget should be a name that the processing application will recognize. An abstract identifier for an SGML or XML document, DTD , or external entity. To continue the cooking metaphor, raw data is just a set of ingredients. It's up to the processor to select appropriate elements, arrange them for display, and add required presentational information.
Vendors often have proprietary stylesheet formats as well.
In SGML , a local, system-dependent identifier for a document, DTD , or external entity. Usually a filename on the local system. In XML , a system identifer is required to be a URI . URI Uniform Resource Identifier, the W3C 's codification of the name and address syntax of present and future objects on the Internet. URI is the umbrella term for URN s, URL s, and all other Uniform Resource Identifiers.