Mike,
If your choice elements are sequence elements Mendix does not support these as not the complete xsd standard is supported.
See https://docs.mendix.com/refguide7/xml-schema-support for supported elements.
If both choices are needed then it could be an option to create multiple xsd's/export schema's to allow for the different options and determine in the microflow logic which export mapping to use.
Joshua and Erwin,
Thanks for your input!
I discovered the error message comes from the following XSD structure:
Eliminating the highlighted maxOccurs eliminates the warning I received in the Modeler when I imported this XSD file.
I thought this might be of use to someone else in the community.
Mike