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DocBook and the DocBookStylesheets are 100% free software, meeting all terms of the GNU Free Software Definition, the Debian Free Software Guidelines, and the Open Source Definition. That means they're free not only in the sense that they're no-cost, but also in the sense that you're free to use them for any purpose, modify them in any way you want, and distribute your modified versions, without unnecessary restrictions and without needing to ask anybody for permission.
However, you are asked to distribute modified version responsibly, meaning that you should name and label them variants so that users will not confuse them with the standard versions.
As far as specific licenses, the wording of the license distributed with the DocBookStylesheets is pretty much the same as the MIT/X Consortium license, including the "advertising clause", which states that:
''the names of individuals credited with contribution to this software shall not be used in advertising or otherwise to promote the sale, use or other dealings in this Software without prior written authorization from the individuals in question''
Though the license distributed with the DocBookDtd does not closely parallel the wording of any other well-known license, it is a 100% free software license.
Both DocBook and the DocBookStylesheets are also copyrighted. The copyrights ensure that anybody who redistributes modified versions has to do it under the terms of the same 100% free-software license the copyright holders chose, so it all stays free.