Conditional Sections:

A way of including or excluding sections in the DTD is to use conditional sections. Conditional sections are of most use when linked to a parameter entity reference, and may only be used in an external DTD subset. The two conditional section statements are IGNORE and INCLUDE.

<![ IGNORE [

markup declarations

]]>

<![ INCLUDE [

markup declarations

]]>

Example:
<!ENTITY % draft "INCLUDE">
<!ENTITY % final "IGNORE">
<![%draft;[
<!ELEMENT book (title,author,summary)>
]]>
<![%final;[
<!ELEMENT book (comments*,title,author,summary)>
]]>
<!ELEMENT comments (#PCDATA)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT summary (#PCDATA)>

Rules:

Note: