Parameter entities are often used in DTDs to avoid unnecessary duplication, to clarify content models, and to generally make maintenance of the DTD easier. Although parameter entities cannot be used in XML Schema documents (or indeed any XML document), it is possible to use general entities instead. However, the XML Schema standard includes its own (arguably simpler) mechanisms to achieve the same aims, so avoiding the need for entities of any kind.
Shared content models
When a number of element content models are identical, it is not efficient to replicate the model in each element definition (and document model clarity and ease of future maintenance also suffers). Instead, the complex model can be created in isolation, ...