This is a subform that originally was set up to add attributes to items. It also used to provide 'add to list' functionality. In addition, it also used to provide a set boolean mechanism so that users can actively state that attribute x was NOT true.
Need to check with HRO if this form has been superceeded by the more generic sf_attr_bytype form. Looks like she is still using it for Portus despite what it says in the sf_assemblage documentation.