May

An optional definition. Programmers may decide whether or not to comply.

Must

A mandatory requirement. Implementations must comply with these conditions.

Optional

see "May"

Shall

see "Must"

Should

A recommendation. Programmers are encouraged to follow this rule, but they may decide to ignore it.

Supported

A supported feature must be provided by the implementation and behave as specified.

Unspecified

No requirements are specified. The programmer may decide whether or not and how to react if the situation occurs. But the implementation must not depend on a certain behavior of an implementation in an unspecified situation.

DTD

A defintion of the legal structure, elements, and attributes for a valid XML document, that complies with the rules of the DTD.

Element

Part of the document structure defined in the DTD. Each element may have multiple attributes.

Attributes

Part of the document structure defined in the DTD. Basically a key-value-pair.

Document

Refers to a valid XML file, that complies with the rules as defined by the DTD. Each document may have multiple elements.

PCData

Part of the document structure defined in the DTD. It defines that an element may have text content.

Empty

Part of the document structure defined in the DTD. It defines that an element must not have any content.

A reference implementation of this format is given by the Yana Framework. This is a recommendation and not a part of the XDDL definition. This document explains optional elements by the example of this reference implementation. These explanations are informal.