Attributes
The value of attributes may be constrained by the DTD using specific attribute type values. For example, "id" attribute is of type ID, which constrains its value to a string beginning with an alphabetic character. An "id" value must be unique within the EAD instance within which it occurs, that is, no other tag in the entire document can have the same "id" value. Most EAD attributes are of type CDATA, which means that any text characters may be used.