For all levels in a state machine, including the root level, for states with exclusive decomposition the following rules apply:
At least two exclusive states must exist.
A state cannot have only one substate.
The initial state of every hierarchical level with exclusive states is clearly defined by a default transition. In the case of multiple default transitions, there must always be an unconditional default transition.
By Task > Modeling Standards for MAAB > Sstateflow > Check usage of exclusive and default states in state machines