1. Extending SDF
A central part of SDF's philosophy is that documents should be specified in a logical manner. In order to make this possible, SDF has been designed to be easy to extend.
If you do extend SDF and other people need to use your extensions, please try to document things. Within the SDF distribution, the SDF document catalog (doc/catalog.html) provides a link for local documentation (doc/local/) which you may wish to use.
If your extensions would be useful to a wider audience, please consider packaging them into an SDF library or module, documenting the features and notifying people on the SDF mailing lists.
Almost every part of SDF is extensible. As a rule, the most commonly created extensions are new macros and new filters. However, creating new paragraph, phrase and table styles is also quite simple.
Details on how to create these things and other SDF entities are given below.