Tip: Customize a DTD with parameter entities
Now on IBM developerWorks: Tip: Customize a DTD with parameter entities — Creating a Document Type Definition (DTD) and using it to validate your document is a good way to ensure that the data fits a particular structure. In some cases, however, you want to give users some control over the structure. Parameter entities enable you to create a structure that allows a document author to choose from two or more possible DTD structures without giving that person control over the actual DTD.