XML processors can be viewed as being of two types, validating processors and nonvalidating processors. Validation is discussed further in Chapter 4.
Both validating processors and nonvalidating processors detect any well-formedness errors in the document entity, including the internal subset of the DTD. However, their behavior might differ with external entities and the external subset of the DTD.
A validating processor must process the XML document entity, any external entities, and the DTD (both internal subset and external subset). It must access those fully to validate the XML document. In doing so, it detects any well-formedness errors in any physical part of an XML document.
Nonvalidating processors, ...