/dtds/docbook/db30 contains DocBook V3.0.
/dtds/docbook/db31 contains DocBook V3.1.
/dtds/docbk/db315 contains DocBk XML V3.1.5.
/dtds/docbk/sdb3151 contains a simplified DocBk XML V3.1.5.1.
/dtds/isoents contains ISO entity sets needed for the DTDs.
/dtds/usadod contains the CALS table model DTD needed for the SGML DocBook DTDs.