Create a new column in the cart, for examples sake, lets call it "discounted". For items that the discount can apply to, set the value in the add to cart button for the discounted column to 1. for items it does not apply to set the value to 0.
When creating the discount rule, add the following trigger:
Number of Item With a Specific Column Value:
Number of items where [discountable] has a value 0f  is [>] a value of 
for the calculation, use:
based on Items with a specific Value:
Total Of [Totalprice] where [discountable] has a value 0f  [times] [0.1]
for a 10 percent discount.