One year at a time. For instance, if my use year runs from June 2014 to June 2015, and I do not use all or some of those points, I can bank them into my June 2015 to June 2016 Use Year, and use them with the points native to that use year to reserve stays that begin on June 1st 2015 or later. Those points could not be banked again forward another year. However, if I were not to use all the points "native" to the use year June 2015/16, those points could be banked forward. When booking, DVC member services always uses banked or borrowed points first, as they are they least flexible. Banking decision must be made by, as I recall, the 10 month mark, so for June Use years, the deadline is March 31st.
Similarly, when one borrows points from the next use year into the current use year, they must be used that year and can not be banked back forward again. And also once again, that would not prevent one from banking unused "native" points that were left over.