The underlying core, SMACH, is lightweight with good logging and utility functions. It has two fundamental interfaces:

• States: States are what you want the robot to exhibit as a behavior or simply execute an action. They have potential outcomes as a result of the execution of actions. In ROS, the behavior or actions are defined in the execute() function, which runs until it returns an identified outcome.
• Containers: Containers are a collection of states. They implement an execution policy that can be hierarchical, execute more than one state at a time (concurrent), or execute the state for a certain period of time (iterative):

Let's consider the preceding diagram and look into SMACH concepts in a bit more detail.