An example

In the DTD subset we write:

 <!ENTITY % TEI.prose "INCLUDE">
<!ENTITY % biblStruct "IGNORE">

The prose tagset within the TEI dtd looks like this:

 
<!ENTITY % TEI.prose "IGNORE">
<![ %TEI.prose [

 .... lots of other declarations ...

<!ENTITY % biblStruct "INCLUDE">
<![ %biblStruct [
  <!ELEMENT biblStruct .... >
  <!ATTLIST biblStruct .... >
] ]>
  ... yet more declarations ....

] ]>

In SGML (and XML) the DTD subset declaration always wins...