To understand your December use year better, make yourself a chaet like this:
2004 use year points are good for vacations happening between 12/1/2004 and 11/30/2005.
2005 use year points are good for vacations happening between 12/1/2005 and 11/30/2006.
2006 use year points are good for vacations happening between 12/1/2006 and 11/30/2007.
etc.
So if the resale says "Hilton Head 225 points (December) - 40 pts. remain for 2004. and 225 for 2005", it means that there are 225 points in the contract, there are 40 points remaining in the 2004 use year and 225 points available for the 2005 use year.
You can bank up to 100% of your annual allotment of points into the next use year if you do it before the end of the 6th month following the start of the use year, up to 50% if you do it before the end of the 9th month and up to 25% if you do it before the end ofg the 10th month.
For a December use year, the 100% banking deadline is the end of May.
For a December use year, the 50% banking deadline is the end of August.
For a December use year, the 25% banking deadline is the end of September.
Since ther are 40 points left in the 2004 use year and the contract is for 225, you could bank the 40 points as long as you did it before the end of September, 2005. 40 /225 is less than 25%.
If you bank the 40 points, they become valid for stays in your 2005 use year - which runs from 12/1/2005 through 11/30/2006. If you don't use those 40 points before 11/30/2006, they expire and you lose them. You can only bank points once.
HTH - I know it was a lot of words. As Diane said, December use years are tricky to understand at first.
Best wishes-