1) ‘Notify till stable’ will continue sending alerts at intervals as long as the counter is in ‘critical’ condition. If you don’t select this, you will get only one mail for the same critical event, no matter for how long the critical condition lasts.
2) ‘Notify when stable’ will send a mail when a critical event stops being critical (the value of the counter becomes lower than what is defined as ‘critial’). If this is not selected you will not be alerted hen a critical condition becomes non-critical.
.. so in my understanding they are independent.
Did you experience anything not in accordance with the explanation above?