This is the Extensible Style Sheets Language developed by W3C primarily to control the way information is presented on screen. XSL also provides some key document-handling facilities beyond styling. For instance, transformations, re-arrangement of elements, extract of a table of contents from a document. These forms may be linked to local database systems. There is no simple answer to this question.
Arguments vary depending on the speaker.
The general consensus however is that XML is the most promising approach to move EDI technologies to the Internet. Another argument is that XML is versatile and benefits from a large industry support. All these are to be built with the same core technology. No symbol means must occur once only.