The i2b2 XML schema consists of three XSD files:

i2b2.xsd

This schema is not used directly to create i2b2 messages, but is included in the i2b2_request.xsd and the i2b2_response.xsd.
It defines the <i2b2:request> tag, which includes the <message_header> tag.

i2b2_request.xsd

This schema is used for validating i2b2 request messages.
It defines the <i2b2:request> tag, which includes the <message_header> tag.

i2b2_response.xsd

This schema is used for validating i2b2 response messages.
It defines the <i2b2:response> tag, which includes the <message_header> tag.