I probably made it too complex so let me do it by example. You have an October use year. The October 2017 use year runs from Oct 1, 2017 to Sep 30, 2018. Let us assume you want to take a trip from September 26, 2017 to October 3, 2017 and you want to use only Oct 2017 use year points for that trip:
1. The nights of Sep 26 through Sep 30, 2017 fall within your Oct 2016 use year which runs Oct 1, 2016 to Sep 30, 2017. Oct 2017 points can be use to book those five nights in Sep 2017 only if you borrow those points. When you call or go online to make the reservation is irrelevant to that issue.
2. The nights of Oct 1 and 2, 2017 fall within your Oct 2017 use year and thus your Oct 2017 use year points will be "current" use year points for those dates.
3. You could call MS today, Oct 26, 2016, and make the entire trip reservation for that Sep 26 to Oct 3, 2017 trip. If you use only Oct 2017 points for the trip, then all points needed to reserve the nights of Sep 26 through Sep 30, 2017, will be borrowed to book those dates. All Oct 2017 points needed to book the Oct 1 and Oct 2 nights of the trip will be "current" use year points for those two nights. The points borrowed for the Sep nights will always remain borrowed. Thus, if you cancel the trip, they will stay in your Oct 2016 use year and will have to be used for a trip in Sep 2017 or before.
4. Unlike calling MS, the online system does not allow you to make a single reservation that has applicable nights in two different use years. It is a system limitation not a
DVC rule limitation. If you wanted to make that reservation for Sep 26-Oct 3, 2017 online, you would have to book two separate reservations. You could today book the five nights of Sep 26 through Sep 30, 2017 online, using borrowed points from your Oct 2017 use year, because you are 11 months out from Sep 26, 2017. But you cannot book the second reservation, Oct 1 and 2, 2017 online today; you cannot book that reservation until Nov 1, 2017, 11 months out from Oct 1, 2017. After you have made those two reservations, you can then call or email MS and have MS merge the two reservations into one.