Default Values

If an optional attribute is not present on a given element, a default value may be provided to be passed by the XML parser to the client application. The following table shows various forms of the attribute default value clause and their meanings:

Default value clause	Explanation
#REQUIRED	A value must be provided for this attribute.
#IMPLIED	A value may or may not be provided for this attribute.
[#FIXED ] "default value"	If this attribute has no explicit value, the XML parser substitutes the given default value. If the #FIXED token is provided, this attribute's value must match the given default value. In either case, the parent element always has an attribute with this name.

The #FIXED modifier indicates that the attribute may contain only the value given in the attribute declaration. Although redundant, it is possible to provide an explicit attribute value on an element when the attribute was declared as #FIXED. The only restriction is that the attribute value must exactly match the value given in the #FIXED declaration.