An unclear point with W3C XML 1.0 Specification

{{ //2.3 Common Syntactic Constructs, XML 1.0 Third Edition
//http://www.w3.org/TR/2004/REC-xml-20040204/#sec-common-syn

Literal data is any quoted string not containing the quotation mark used as
a delimiter for that string. Literals are used for specifying the content of
internal entities (EntityValue), the values of attributes (AttValue), and
external identifiers (SystemLiteral). ___Note that a SystemLiteral can be
parsed without scanning for markup.___

}}


I wonder what the last sentence really wants to say. 

I think I must have missed certain backgroud knowledge about it.

Could you give me some explanation ?

Thank you!


Collin