>a CDATA section defined in a DTD
I think you are confusing different concepts here.
CDATA sections are not defined in a DTD. However, DTDs do use very similar keywords for two other (but different) concepts: CDATA is one of the attribute types available (the others are ID, IDREF, NMTOKENS, etc), and #PCDATA can be included in a content model for elements to indicate that it can contain text (rather than or as well as child elements). Both of these concepts are quite similar to the XML Schema concept of xs:string.
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference