When you buy your contract you are allotted (ex) 200 points in each and every UY. The UY month is when your particular UY starts.
You are allowed to use whatever points in the UY in which you are traveling. And you have to book within the booking rules.
If you are booking a trip that falls in your 2021 UY, you can use:
2020 banked points (if any)
2021 UY points (not yet banked)
2022 UY points (if borrowed / subject to rules)
Basically, you have access to UP TO 3 years of points at once. Right now borrowing is limited to 1/2 your uy points, so a total of 500 points could be used in 1 UY on a 200 pt contract. (in normal times it would have been 600)
You have until the end of your 8th month of your UY to bank that UYs points. Once you bank them, they can no longer be used in the current UY and will expire at the end of the UY you banked them into.
Lets say you buy
DVC tomorrow in an Oct UY and want to book for November 2021. You have to bank your 2020 points FIRST if you want to use them for your 2021 trip. You have until May 31 to bank them. If you don't bank them by May 31, 2021 they expire September 30, 2021. You can still book November 2021 without banking those points, but then the system will take your 2021 points.
Does that make sense?
So, let's go back and say you get your points tomorrow, bank them and book for November as planned. Then you get really excited and decide you can't wait and want to take a trip in May (2 months from now) instead. You cannot use those points you banked because they are now in the 2021 UY and a May 2021 trip is in your 2020 UY. They cannot be moved again.
Still make sense?
But you still want to take a trip in May dang it!!! You would have to borrow your 2021 points (not the banked 2020 points). Currently, you can only borrow 100. In normal times, you could have borrowed all 200. But those 2020 points that are now in your 2021 UY will expire if you don't use them by September 2022.
But now you can see how if you start changing plans after banking or borrowing, your points get a little messy.