The W3C XML Schema language in particular can be quite verbose, while a DTD can be terse and relatively easily editable.
Likewise, WXS's formal mechanism for associating a document with a schema can pose a potential security problem.
Both allow for a degree of modularity in their languages, including, for example, splitting the schema into multiple files.
And both of them are, or can be, defined in RELAX NG does not have any analog to PSVI.
Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.
The mechanism for associating an XML document with a schema varies according to the schema language.
XSD schemas are conventionally written as XML documents, so familiar editing and transformation tools can be used.
As well as validation, XSD allows XML instances to be annotated with type information (the Post-Schema-Validation Infoset (PSVI)) which is designed to make manipulation of the XML instance easier in application programs.
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself.
These constraints are generally expressed using some combination of grammatical rules governing the order of elements, Boolean predicates that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as uniqueness and referential integrity constraints.
DTDs are perhaps the most widely supported schema language for XML.