Your task lists can be customized to sort in a number of different ways. With a Pro or Pro Plus account, you can select three sort criteria. With a basic account, you can set two. The default criterion used is "Importance", but you can sort them by Priority, Due Date, and just about anything else. Just click "Sort" above your task list, and then select your desired criteria, then click the Sort button. Alternatively, you can quickly sort by any field that has a column in Grid view by clicking on the header of the column. I hope that was what you were asking about. If you mean how are they sorted beyond that, I believe it is fairly random, and there is no way to manually rearrange them.