XML - HTML and XHTML: The Definitive Guide - page 845

Here is an external general entity declaration:
<!ENTITY boilerplate SYSTEM "http://server.com/boilerplate.txt">
It tells the XML processor to retrieve the contents of the file boiler-
plate.txt from server.com and use it as the value of the boilerplate en-
tity. Anywhere you use &boilerplate; in your document, the contents of
the file are inserted as part of your document content.
Here is an external parameter entity declaration, lifted from the HTML
DTD, which references a public external document:
<!ENTITY % HTMLlat1 PUBLIC "-//W3C//ENTITIES Latin 1 for XHTML//EN"
"xhtml-lat1.ent">
It defines an entity named HTMLlat1 whose contents are to be taken
from the public document identified as -//W3C//ENTITIES Latin 1 for
XHTML//EN . If the processor does not have a copy of this document
available, it can use the URL xhtml-lat1.ent to find it. This particular
public document is actually quite lengthy, containing all of the general
entity declarations for the Latin 1 character encodings for HTML. [*] Ac-
cordingly, simply writing this in the HTML DTD:
[*] You can enjoy this document for yourself at http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent .
%HTMLlat1;
causes all of those general entities to be defined as part of the language.
A DTD author can use the PUBLIC and SYSTEM external values with general
and parameter entity declarations. You should structure your external
definitions to make your DTDs and documents easy to read and under-
stand.