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Revision and Maintenance Policy
A backwards-incompatible change is a change to any ELEMENT or ATTLIST declaration (or any parameter entity or other construct that such a declaration references, directly or indirectly) that results in a restriction of markup choices, such that some or all document instances conforming to previous versions might not conform to the new version.
The Davenport Group's policy is not to make backwards-incompatible changes in minor revisions (x.n to x.n+1); such changes shall be made only in major revisions (x.n to y.0). There will be no more than two major revisions of DocBook per year, and in practice there will probably be fewer than two. The Davenport Group will always give warning of backwards-incompatible changes at least six months in advance.
Accordingly, some future backwards-incompatible changes are signaled in "FUTURE USE" comments in the DTD. The future version in which the change will take place is indicated in each comment.
Note: Changes to parameter entities that affect the DTD's customizability—for example, changes to parameter entity names—are not restricted to major revisions. Thus, DocBook customizers should review each new minor revision and may need to make changes to their customization layers.
See Chapter 3 for lists of changes made in recent and planned versions of DocBook.