Associating a schema with a document, or changing the current association
Using the same schema with several documents, without having to reload it
Validating documents against a schema
Validating document parts against a schema
Retrieving information from a schema (e.g. default values and attribute types
Create new schemas (like the DOM creates new instance documents)
Save an in-memory schema to a file.
Modify in-memory schemas
Provide the guidance necessary so that valid instance documents can be modified and remain valid.