The Default Declaration
The default declaration can consist of any valid value for your attributes, or it can consist of one of three predefined keywords: #REQUIRED, #IMPLIED, or #FIXED. The #REQUIRED keyword indicates that the attribute must be included with the element and that it must be assigned a value. There are no default values when #REQUIRED is used. The #IMPLIED keyword indicates that the attribute does not have to be included with the element and that there is no default value. The #FIXED keyword sets the attribute to one default value that cannot be changed. The default value is listed after the #FIXED keyword. If none of these three keywords are used, a default value can be assigned if an attribute is not set in the XML document.