If you purchase direct from Disney, and purchase less than 150 points, the new add-on will automatically become your same use year month, you cannot get a different month. If you purchase a minimum of 150 points then you must specifically state that you want a different use year month in order to get one. Otherwise it will be the same use year month.
If you purchase resale, the use year month will be whatever the use year month is that the resale contract already has. You cannot change it. If the contract by chance has the same use year month as your existing contract, then you really want the contract treated as an add-on to your existing contract. Disney will not necessarily do this automatically. You need to verify with Disney that this is wht you want to do. If you don't, Disney may not treat it as an add-on but rather as a new contract. Your resale agent can help you with this.
Difference between contracts with same use year month. (Regardless of resort)
Same Use Year Month (Add-on Contract)
Points are combined for banking purposes. Thus if you have a 150 point OKW contract and a 50 point SSR add-on, it's treated as a 200 point contract. Suppose you use all your 150 OKW points and none of your 50 SSR points. At the 50% banking deadline, you can still bank all 50 SSR points since you can bank 50% of 200, which is up to 100 points.
At the 7-month window, points can be combined in any manner you want to make a non-home resort reservation. Suppose you wanted two nights at BWV and it costs 36 points. You have 25 points left in your OKW account, and 50 points left in your SSR account. You could use all 25 of your OKW points and 11 of your SSR points, or any combination you wanted to make that reservation. This would leave you with 0-points in your OKW account, and 39 points in your SSR account.
Different Use Year Month (This applies even if the same resort)
Points are not combined for banking purposes. So in the example from above, at the 50% banking window, you could only bank 25 of those 50 SSR points.
Points cannot be combined to make a reservation. Suppose as in the example above, you had 25 OKW points left and 50 SSR points left and wanted to make that same 36 point two night reservation. (At 18 pts/night). You would have to use 18 of your 25 OKW points for one night, and make a separate reservation for the 2nd night using 18 of your SSR points. You then have to link the two reservations so you won't have to check-out/check-in again. However, your account balances are now 7-points in your OKW account, and 32 points in your SSR account.
Hope this helps.