My question is simply: under what conditions will empty tags of the
form <MOM></MOM> pass schema validation?  Of course, the mirror
question is: under what conditions will empty tags fail validation?
The former seems to be an easier question to answer.

XML files will arrive from around the world and must be schema
validated before further processing and loading into a database, so I'm
trying to foresee the various layouts that might be submitted.  I can
anticipate suppliers starting with a template, filling in needed
elements, and sending the file with empty tags in conditional segments
with mandatory and conditional elements.  I understand the role of
restrictions, but there are about a dozen record types, dozens of
segments, and hundreds of elements (some of which are sometimes
mandatory, sometimes conditional, and sometimes
conditionally-mandatory).  One schema is 230 pages.

I already created a test file where a conditional segment had empty
tags and validation failed. 

Thanks