The Priority of the quests is calculated based on the map.
If a quest is above an item on the same level, then its priority is higher.
If a quest has sub-quests then you need to complete them first.
You can move a set of quests under another quest to reorder your priorities.
Or you can change the priority of the quest at the same level by drag and drop.
The sub-quest and the priority system allow you to group your tasks and set priorities at the same time. That makes the quest's priority gradually detailed but easy to handle. You don't have to hustle the high, medium, and low-priority decisions.
