On processing, these entities expand to the contents of the files.
In the DTD:
<!ENTITY ChapTwo SYSTEM "sgmlmkup.txt">
In the document:
<body> &ChapOne; &ChapTwo; ... </body>
External entity specifications can refer to SYSTEM or PUBLIC identifiers.
<!ENTITY mteO-ro PUBLIC "-//MTE//TEXT CES1 1984//RO">
If FORMAL=YES then PUBLIC identifier must be properly structured.