System Architect objects are sent to formal modules in IBM Rational DOORS. You can specify that System Architect model artifacts are sent to multiple IBM Rational DOORS modules. You specify what IBM Rational DOORS module to send artifacts to by associating artifacts with a IBM Rational DOORS transfer unit definition in System Architect. The transfer unit is a definition type in System Architect. When you port System Architect artifacts to IBM Rational DOORS, you specify what IBM Rational DOORS surrogate module a IBM Rational DOORS transfer unit definition will be associated with. There is a one-to-one relationship between a IBM Rational DOORS transfer unit definition and a IBM Rational DOORS surrogate module.