If you have a contract with 300 pts per UY, then those 300 pts are valid for stays within that 12 month period.
You can bank or borrow to move pts up or back 1 UY. You can only move a point one time, so if you move a point into a new UY with banking or borrowing, it now has to be used to complete a stay within that UY or else it will expire and disappear into the void.
Banking and borrowing allow you to use 3 years of pts within 1 UY.
So if you are booking a stay for August 2018 with a Feb UY, you would be using current 2018 UY pts with access to banked and borrowed pts from the prior and next UY if you want.
If you are booking a stay for Aug 2018 with an October UY, that would fall in your 2017 UY (Oct 1 2017 to Sept 30 2018), so you would be starting with current 2017 UY pts, with access to banked 2016 and borrowed 2018 UY pts if you want.