In general, all rules on subtyping apply for StateMachine types as well. Some additional rules apply for StateMachine types. If a StateMachine type is not abstract, subtypes of it shall not change the behaviour of it. That means, that in this case a subtype shall not add States and it shall not add Transitions between its States. However, a subtype may add SubStateMachines, it may add Transitions from the States to the States of the SubStateMachine, and it may add Causes and Effects to a Transition. In addition, a subtype of a StateMachine type shall not remove States or Transitions.