What happens to unused data each cycle
The available data is reset to the amount you are paying for according to the plan you chose.
One cycle is not affected by the previous one, independently of whether you used or not all the available data in the previous one.
I hope this makes sense to you, please let me know if you have any other questions!
If you are interested in saving data (in order to maximize your usage each month) we highly recommend trying this new experimental feature that our Forum team is testing: Announcing the Data Saver Feature Experiment!
So far our testers are very happy with this Data Saver feature