Use year determines when your points are issued each year (first day of use year) and by when you must bank them. You have until the end of the 6th month of your use year to bank 100% of total annual allotement of points. From the beginning of the 7th month to the end of the 9th month you can bank up to 50% of the total and by the end of the 10th month 25%. Understand that means if you bank 50% by the end of the 6th month you can bank no more thereafter; likewise if you have banked 25% by the end of the 9th month you can bank none in that 10th month.
Use year has nothing to do with when you can go (anytime of year) or when you can call to make ressies (always 11 months in advance at home resort, 7 at others). The only impact use year can actually have relates to your canceling a trip and that impact results from the banking deadlines.
Assume you have a Feb use year, meaning it runs from Feb 1 to Jan 31 and you schedule a trip for late Jan at the end of your use year using pointts issued in that use year. Something happens and you need to cancel at the beginning of December. When you do so you will be beyond all banking deadlines and thus you won't be able to bank any of those points and they must be used by the end of Jan.
Now assume you have a December use year and schedule that Jan trip. That trip is now at the beginning of your use year. If you cancel in early December you will still be able to bank all the points or have 11 months to use them (your choice).
As a result, if you have a usual time of year that you go then some recommend that you get a use year starting month that is no more than 6 months before the time you usually go. Thus, if you must cancel you will always have time to bank all points.