| The structure of DTDs is shown in the MSR document as DTD diagrams (see convention in DTD diagrams). Figure 1: convention in DTD diagrams The meaning of the symbols is:
PCDATA | Processable Character Data (PCDATA) Data that consists of zero or more characters of both text and markup. PCDATA is a declared content keyword. PCDATA is used to indicate that all markup delimiters defined in the SGML declaration will be recognized by the parser as markup in the given element rather than data characters.
RCDATA | Replaceable Character Data (RCDATA) is data that consists of zero or more characters, in which references to substitutions are not recognized (i.e. RCDATA may contain text and entity references, but no sub-elements). See also: CDATA PCDATA.
CDATA | Character Data (CDATA) consists of zero or more text characters, where no markup of any kind is recognized. CDATA is an SGML term. Note that character references are allowed in a CDATA entity (substitution) but not in CDATA content.
ANY | a terminal type indicating that the object may contain text or any element defined in the model.
EMPTY | a terminal type keyword used to indicate that there is no data (i.e. no content, sub-elements or end-tags) for the object allowed in the document instance. This keyword is often used to describe elements that are placeholders or are pointers to external or system-generated data.
One | indicates that the element or the element group occurs exactly once
ZERO-OR-ONE | indicates that the element or the element group is optional
ONE-OR-MORE | indicates that the element or the element group occurs multiple times but at least once
ZERO-OR-MORE | indicates that the element or the element group occurs multiple times but also can be missed (optional)
ORDERED | a connector used to specify that the sibling objects must appear in the document in the order shown in the model
UNORDERED | a connector used to specify that the sibling objects can appear in any order in the document.
SELECTION | a connector used to specify that only one of the sibling objects can appear in the document.
ELEMENT | indicates a single SGML structure element
COLLAPSED | indicates, that the content of the element is not displayed here