Conditional sections
Portions of a DTD may be identified as optional segments, which can be easily included or excluded from processing to build alternative document models. This facility is provided using conditional sections. Once a segment of the DTD is marked as a conditional section, the content is made visible or invisible to XML processing by changing a single keyword.
It should be noted that conditional sections can only be used in the external subset of the DTD. It is not possible to use them in a document's internal subset, or in any external entity referred to from this place. It is also not possible to use them in document content.
SGML Note
The included and ignored section markup cannot be used in a document instance. In XML, this ...
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