Facial selectivity of iodonium formation in an iodolactonisation
Here, it does not matter which face is preferred. The formation of the iodonium ion is reversible and it can be formed on both faces. However, the iodonium ion can only be attacked by the carboxylate if it is formed on the opposite face from the carboxylate, which is the bottom face in your diagram. Attack on the top face is said to be unproductive because it cannot lead to iodolactonisation.