There are discount and promotion eligible for service plan. How will they be applied during service plan purchase?
Only one promotion or discount can be applied to a subscription. If service plan sales are eligible for both discount and promotion, promotion is applied to sales while discount is ignored.
More specific discount has priority over less specific ones. For example you have created three discounts: first - applicable to all service plans, second and third - applicable to a service plan and to one of its subscription periods respectively. Therefore, theoretically, you have three discounts applicable to one service plan. However, only one of them is applied - the most specific of three. In our example it is the discount on particular subscription period. When customer subscribes to that subscription period, he gets the third discount only. When customer subscribes to any other subscription period of the plan, he gets the second discount. He won't get first discount in any case. Such mechanism may come in handy when you offer general discount on all of your plans and need to single out few particular plans (subscription periods) and give some distinct discount on them.
There is a feature request with id #PBA-43010 ("Ability to apply Discount, Promotion or both") to modify the existing behaviour.