use case:
if user(A) signs up user(B) & himself for Event(E) and user(B) belongs to a role that gets 10% discount and user(A) doesn't
then the discount should be applied to only userB's order

example:
if the event costs 100$
user A adds the event to the cart with quantity 2
and there is a 10% discount for user B
so user A will pay 100$ + 90$

Discount module: http://drupal.org/project/uc_discount

to make this work uc_signup should provide signed up user(user B) as argument for the conditional action (Condition: Check the user's roles)