The S130 stack has multiple activities, called timing-activities, which require exclusive access to certain hardware resources. These timing-activities are time multiplexed to give them the required exclusive access for a period of time, this is called a timing-event. Such timing-activities are BLE role events (central roles, peripheral roles), Flash memory API usage, and Radio Timeslot API timeslots.
If timing-events collide, their scheduling is determined by a priority system. If timing-activity A needs timing-event at a time that overlaps with timing-activity B, and timing-activity A has higher priority, timing-activity A will get the timing-event. Activity B will be blocked and its timing-event will be rescheduled for a later time. If both timing-activity A and timing-activity B have same priority, the timing-activity which was requested first will get the timing-event.
The timing-activities run to completion and cannot be preempted by other timing-activities, even if the timing-activity trying to preempt has a higher priority. This is the case, for example, when timing-activity A and timing-activity B request timing-event at overlapping time with the same priority, and timing-activity A gets the timing-event because it requested it earlier than timing-activity B. If timing-activity B increased its priority and requested again, it would only get the timing-event if timing-activity A had not already started and there was enough time to change the timing-event schedule.