The API has a number of improvements for wokring with multiple sites with discounts and products.

• When switching the site of a cart, discounts and products will switch to the site-specific alias for that product in the cart. Previously they would stay the product/alias that was originally inserted into the cart.
• Discounts on the main product record for a product are now also applied to aliases. The opposite is not true, discounts specifically on aliases will not apply to other aliases or the master product.
• A new option "Multisite" in admin alows you to specify that a discount can contain products from more than one site.