Gilles and Roberto have been helping me off list. I found this snippet of text from the Data Mapper Guide helpful. The 3 bullet points were the kind of information I was looking for:
" iBATIS DataMapper support for implementing object-oriented inheritance (subclassing) your object model. There are several implementation options for mapping entity classes and subclasses to database tables: * table per class hierarchy * table per subclass * table per concrete class You can use the most efficient mapping strategies from a SQL and query performance perspective. The Sql result must define one/some column(s) in your result that will serve to identify which resultMap should be used to map the table row. In most case, you will use one column value to identify which resultMap. "
