Some projects will have base xml schemas that may be used across many child xml schemas. In order to avoid having to build these base schema libraries multiple times, you can create a 'Super Schema' that includes all of the child schemas.
When you generate code from the 'Super Schema' then the base libraries will only be generated once and can then be accessed from each of the generated child libraries.
Please see the help file for working with multiple schema:
Working With Multiple Schemas