Assignable relationship field mappings are used, for example, in an explicit field assignment where fields with different names are matched. In Rational Rose, assignable field mappings are created by adding keys/qualifiers to one of the Association roles.
In Rational Software Architect, assignable field mappings are maintained on the General tab of the assignable relationship's properties. The mapping is maintained by defining the fields involved in the Role option of each class. Additional fields can by specifying using a comma separated entry.