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That's a basic, but not exhaustive, overview of how the module work. We didn't discuss how obsgroups are implemented, or about the various classes in the schema package, among other things. Check out the javadocs for more information.
Custom Tags
For a good example of how to add a custom tag to the valid list of htmlformentry tags from another module (or, in other words, how to register a custom tag handler from another module), see the htmlformflowsheet module.