for us: 
1* mildly hot

2* medium hot
3* significantly hot

4* aggressively hot
5* hottest spice level available, given available ingredients, while maintaining balance of flavor

The perception of spiciness is as subjective as it is difficult to quantify. Another way of putting that is asking the question: how long is a piece of rope? We strongly recommend ordering at least one level below what you would normally order. additional spice can always be brought on the side.