What must every orthogonal region always have?

The requirement for every orthogonal region to have only one active state at all times is fundamental to maintaining the integrity of state behavior in state machine modeling. Orthogonal regions, which are used to represent concurrent states, allow a modeler to encapsulate independent behaviors that can occur simultaneously. However, within each orthogonal region, it is essential that exactly one state is active at any given time. This rule ensures clarity and predictability in how transitions and events are handled, preventing ambiguity about which state is responsible for responding to inputs or transitioning to other states.

When adhering to this principle, it becomes manageable to define, analyze, and implement the behavior of the system being modeled. The fact that only one state can be active at a time simplifies the decision-making process within that region, allowing other regions to operate independently without interference from simultaneous active states. This characteristic is crucial for maintaining a clear state management approach, especially when one is dealing with complex systems that involve multiple interacting components and behaviors.