One to Many is a new feature in Better Together 2.5. It is best explained by example. Suppose you have the following discount structure:
$this->add_prod_to_prod(27, 3, "%", 50);
$this->add_prod_to_prod(27, 25, "%", 50);
Where 27 is the HP LaserJet Printer, 3 is the Mouse, and 25 is the keyboard.
Now suppose you have 1 product 27, and 2 product 3 and 2 product 5 in your cart.
The way Better Together has always worked is as follows: it would run the first rule, see that item 27 was in the cart, see that item 3 was in the cart, and discount item 3. Now item 27 would no longer be permitted for use in discounts, and that would be the end of the discount.
This is shown, using Discount Preview, by the following screenshot. Note that the discount is $20.00, 50% of the the price of the mouse (item 3).
Better Together One to Many = 0 (Classic Mode)
Some people wanted the option of discounting both item the mouse and keyboard, even though there was only one printer in the cart. In Better Together 2.5, this can be done by setting the configuration option One to Many to 1 (in Admin->Modules->Order Total->Better Together). Note that the discount is now $54.99, which is half the price of the mouse (item 3) and the keyboard (item 27) combined.
Better Together One to Many = 1
And finally, some people wanted unlimited mice and keyboards to be discounted, which can be done by setting One to Many to 2 (in Admin->Modules->Order Total->Better Together). Note that the discount is now $109.98, which is half off the price of two each of the mouse and the keyboard.
Better Together One to Many = 2
Note: If you were running Better Together 2.4 or older and you download version 2.5 or higher, you must go to Admin->Modules->Order Total and Remove and Re-Install Better Together to get access to this feature.