XML Schema - ref tag not supported in export mappings

I have an XSD file that is generating a warning when I try to create an XML schema in the Modeler.  The error message is as follows:

Type of element 'http://www.---------.com/data/imports/invoice:Tariff': This 'Choice' element is not supported in export mappings.

The XSD file looks like this:

Note the inv namespace highlighted.

Looking at the Tariff element, if I exclude the ref elements highlighted below, I don't get the error message in the modeler:

These ref elements point to other parts of the the same XSD file.

Has anyone encountered this before?  Any pointers about how to work around this?

Thanks!

Mike