UY is confusing and December has got to be the worst.
Because I have literally been known to have 10 minute IRL conversations where we just keep saying the same thing and aren't understanding each other until someone says it yet another way, maybe this slightly different way will help.
Here's my scenario: 0 points 2016, 6 points 2017 and all of 2018. I need to bank those 6 into 2018, thus eliminating all of 2017. So now I only have 2018 to play with. Since that is the case, can I create a reservation in June using all of 2018 and borrowing 2019 or is the case that I'm already borrowing 2018 as it is? I need to bank by July but create the reservation starting in June (not my home resort). The reservation is for January 2018. I'm sorry if I'm being frustrating I just know I didn't explain clearly the first time. Thank you for your patience.
You're taking a trip in January 2018.
2017 Dec UY is "naturally" for trips from 12/1/2017 to 11/30/2018.
2018 Dec UY is "naturally" for trips from 12/1/2018 to 11/30/2019.
Your 6 2017 points naturally sit in the UY for your January 2018 trip. You don't do anything with those. They are fine.
Your 2018 points naturally do NOT sit in that UY, and therefore need to be borrowed from the 2018 UY and put into the 2017 UY. In that way they can be used for your January 2018 trip.
2019 UY is naturally for trips from 12/1/2019 to 11/30/2020. The only thing you can do with those is borrow them into the 2018 year (for a trip 12/1/2018 to 11/30/2019) OR bank them into 2020 (for a trip 12/1/2020 to 11/30/2021).
Your 2019 points are not usable for your January 2018 trip.
I truly feel for December UY people! It's rough enough sometimes with a Feb UY, and (since I believe there is no January UY) you can't get closer to matching the calendar year than that! You've got my empathy, that's for sure!
(sure hope I said everything right)