There are two types of contracts: A Master Contract, and an Add-On Contract.
Your first contract is the Master. It determines your use year month, and you get a membership number associated with that contract.
If you purchase additional contracts, if they have the same use year month, they become add-on contracts. They remain under the same membership number and the same contract number as the original master contract, but with a different extension. Example, your master contract is 12345.000 and your add-on contract is 12345.001
If they have the same use year month, the 2nd contract becomes an add-on, whether purchased directly from Disney or via resale.
If it's a different use year month, it becomes a new master contract and a new membership number is associated with it.
Two master contracts are totally separate from each other. As different as your contract is from mine. Master contracts must be managed individually.
An Add-on contract effectively combines the points between the master and the add-on, whether they are the same resort or separate.
Banking percentages apply to the combined point total for a master contract and all it's add-on contracts. Suppose you have three 100 point contracts, each at a different resort. Effectively it is treated as a single 300 point contract for banking purposes. Suppose the contracts are at OKW, BWV, SSR respectively, and in a given year you've used all 100 OKW points and all 100 BWV points, but none of the 100 SSR points. You're in your 50% banking window. You may still bank all 100 SSR points since that's less than 50% of your total. (300 points times 50% = 150 points)
If instead all 3 were individual master contracts, then at the 50% window you could only bank 50 of your SSR points. (50% of 100 = 50)
The other difference is that at 7-months, all points from a master plus add-on contracts may be combined in any manner you want for making a reservation. Suppose in the above example you wanted to make a 75 point reservation at VWL. If you wanted you could use 25 of your OKW points, 25 of your BWV points, and 25 of your SSR points to make that reservation. You could use points in any combination you wanted.
With separate master contracts, points may not be combined in any way to make a reservation. All reservations must be made separately using points from the contracts separately. (Although one way around some of this is to transfer points from one contract to another, but again this requires a lot more point management.)
Hope this helps.