Along with DocBookDtd itself, DocBook doctypes distributed by the DocBookTechnicalCommittee or DocBookOpenRepository include the following DocBookCustomizations:
SimplifiedDocbook (a strict subset)
SlidesDoctype (an extension for creating slide presentations)
WebsiteDoctype (an extension for creating websites)
JrefEntry (an extension for creating Java documentation)
DocBookLite (A subset used by O'Reilly)