Attr: Tokenized attribute values.

Some clarification please.   In the DOM2 spec of December 10, 1999,
section 1.2 describing the Attr interface [p.57] contains the following
paragraph:

"In XML, where the value of an attribute can contain entity references,
the child nodes of the Attr node provide a representation in which entity
references are not expanded.  These child nodes may be either Text [p.66]
or EntityReference [p.71] nodes.  Because the attribute type may be
unknown, there are no tokenized attribute values."

I don't understand the meaning of the last sentence.  Does it mean that
attribute values MUST be atomic?  Can anyone provide a clearer explanation
and possibly illustrate the meaning with an example?

Thanks in advance.

S.