It is simply this. If you have a February use year, and make a reservation for October, November, December or January, and have to cancel, even if you cancel 31 days or further from arrival, if you cancel after September 30, you can not bank the points. If you cancel at least 31 days prior to arrival and before September 30, you can bank them to next use year.
In other words, if you cancel a January reservation in October, those points have to be used for a stay that must be completed before January 31. Meaning you have a good chance of losing those points. But if you cancel a February stay in October, you can bank the points if you wish, giving you two years to use those points.
No matter what your use year, is it best to avoid booking a stay for nights during your 0% banking window, if possible.