As Doc stated, use year is simply the date you receive your new year's allotment of points. The use year month has two impacts:
1. Banking deadlines are based on your use year month, so the months you can bank 100% of points, 50% of points etc will relate directly to your use year.
2. Reservations can be made at the 11-month (or 7-month) windows. The only requirement is that you will have the points in your account at the time of the actual date of the trip. (They don't have to be in your account at the time of the reservation). Thus your use year is important in knowing on what dates points are actually added to your account in relation to your reservation request. This in turn can affect your banking and borrowing requirements.
For example, suppose you have no 2001 points left, and all your 2002 points will be available, and your use year is July. If you want to make a reservation for June, then you would need to borrow from your July 2002 points to make the reservation. Once borrowed, those points could not be returned so if you cancelled your trip those points could become distressed (in danger of losing them).
On the other hand, if you made a ressie for August, you would simply be using your July points, so if you cancelled that trip, the points remain in your 2002 year allotment.
Hope this hasn't confused you. Basically, it's probably better if your use year begins slightly before the timeframe you would be taking most of your vacations. Then banking and borrowing are not so critical in instances where you have to cancel.
Final Note: If you would never wind up cancelling a reservation, then use year has absolutely no impact.