An alias is a short name given by the user to a URN (the identifier for a namespace).

The XML attributes of a tag are pairs of strings in the format name-value. The second string is contained within quotes. XML attributes serve to characterize the tag.

The XML tag is text delimited by the < and > symbols. This text can be recognized by its name (a set of characters containing no spaces) and its attributes. Example: <person>

Tag definition is the concept that identifies the names of its attributes, the multiplicity of its attributes, and sub-tags contained in the tag. This concept defines a family of tags (we will use UML classes to define tags). Instances of these tags are found in XML documents. Tag definition should be distinguished from tag.

A dependency is a one-way relation between elements. The dependency relationship connects elements within the same diagram.

An element represents a tag in the schema editor The XML elements of a tag A are defined by the set of tags potentially included in A.

The namespace is a set of predefined terms (set of tag definitions, elements and attributes of a document). Inside this space, two tags are not allowed to have the same name.

A group is a set of elements or attributes. It is usefule to constitute groups to order the elements they contain.

Inheritance is a link between two classes, by which the child class inherits all the attributes and elements of the parent class.

References are used to create a relationship between two tags without having to include the referenced tag. For example, if a document contains a list of persons, you can refer to the persons declared in the list without having to include the actual declaration. This eliminates redundancies and facilitates document maintenance.

The concept of overloading is common in object-oriented programming, but it usually only applies to operation methods. It is a mechanism for defining additional properties on an inherited attribute.

The URN is the technical identifier for a namespace. It uniquely describes the namespace.

XML stands for eXtensible Markup Language. This is a language for describing and exchanging structured documents. The first version of XML was adopted by the W3C in January 1998. Users can add new tags to XML. This is why the language is said to be extensible. The ability to add extensions to the language is the key feature of XML.