The atg.commerce.csr.returns.ReturnTools.createReturnItemInstance(CommerceItem) method creates a ReturnItem instance based on the returnItemTypeClassMap property value. If a commerce item type is configured with a ReturnItem class, a ReturnItem instance is created. This method calls the ReturnItem(CommerceItem) constructor and sets the commerce item property.

The default configuration for the returnItemTypeClassMap property is:

/atg/commerce/custsvc/returns/ReturnTools.properties
returnItemTypeClassMap=\
           default=atg.commerce.csr.returns.ReturnItem

When ReturnTools creates a ReturnItem, it will use the type value and create an object of the class you identify.