"Use Year" is simply when you receive your points and determines your banking deadline. If you buy in now, you may have a December UY. This is what I was offered and it confused the crap out of me! I'll try and explain the best I can...
You'll receive your points (say 160 for illustration purposes) for your 2008 UY on December 1, 2008. You can use those points to make a reservation that is completed on or before November 30, 2009. For the current AKV points, your vacation cannot START before May 1, 2009 because that is when your "property" will be open for occupancy. If you are only using, say, 100 points for your trip, you can bank the remaining 60 but it must be done by July 31, 2009. The banking deadline for a December UY is July 31. That means any points not banked by then will expire November 30 of that year.
Now, in 2009, you will receive 160 (+ any banked points) on December 1, 2009. These points can be used for reservations between December 1, 2009 and November 30, 2010. Here's where UY gets a little confusing. If you are planning a trip to AKV for, say, June of 2010, you are actually using points from 2009 UY.
Also, you don't have to wait until your UY begins to make a reservation. If you want to make a January 2011 reservation (with 2010 UY points), you can still make that reservation in February 2010, even though your points won't "be there" until December 1, 2010.
I hope this helps. I had a lot of trouble figuring out a December UY until I just drew it out on paper using arrows and lines.

I "think" I have it now....
PLEASE, experts, help me out! I know that others can explain the importance of UY when planning vacations. I cannot even start to explain that!
EDITED: Oops! I type too slow. 4Pluto answered ahead of me!