There are many other text encodings that predate Unicode, such as ASCII and ISO/IEC 8859; their character repertoires in almost every case are subsets of the Unicode character set.
The specification places requirements on what an XML processor must do and not do, but the application is outside its scope.
The processor (as the specification calls it) is often referred to colloquially as an XML parser.
Disparate systems communicate with each other by exchanging XML messages.
The message exchange format is standardised as an XML schema (XSD). XML has come into common use for the interchange of data over the Internet.
XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used.
Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser.
The characters making up an XML document are divided into markup and content, which may be distinguished by the application of simple syntactic rules.
Generally, strings that constitute markup either begin with the character An element is a logical document component that either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag.
Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.