Basically, the rooms/points at a particular DVC resort are split up over the various use years. If I counted right there are 8 UYs. So the total points available for a resort are divided by 8 and assigned to each UY. Hence there are a limited number of points available for each UY.
So depending upon when the declare the rooms, in a new resort it is possible for a UY to be sold out until more rooms are declared.
In general, Disney assigns you the UY when you are buying your points. My guide didn't really ask me, just gave me August points. You can ask when you are buying your points for a particular UY and if it is available, your guide can give you that UY instead of the one(s) that they are selling at the time.
BLT is being sold with February UY, but my guide said I could ask for a different month if I wanted to. I kept it as Feb. as it works well for us (most our travel falls over spring break and the summer months, so all our points become available each year right before this time.)
As others have said, the guides are offering people Feb UY as the default. However, you can get whichever one works best for you.
Having a UY that falls right before you typically travel is beneficial since it allows you a lot of flexibility if you have to cancel or change a trip at least 31 days out.
With DVC, you can bank your points (all or some) as long as you are in the first 8 months of your UY. When you travel during that time frame, you would still be able to bank points if you cancelled a trip, or changed your reservation and it took less points. It also gives you time to use up borrowed points in case of a cancellation.
Travel habits do change over time and some people travel all different times of the year so UY becomes less of an issue.
For us, we travel in the summer because I am a teacher. So we have a June UY. That gives me until January 31st to bank points. All of my travel would occur during this time so it works out well!
I was told that the Use Year is based on the section of the resort they are selling at that time. There are 8 different use years available and it is ideal to purchase a use year that is prior to when you normally travel. If Disney is offering you a use year that will not work you, you can always ask for a different one that will. The use year should not effect how or when you use your points. You can use your points 365 days a year.