The most commonly used type of basin waste are slotted. The main difference is if your basin has an overflow - which most do. Slotted wastes allow any overflow water to be drained even if the waste is closed. Whereas unslotted wastes are watertight, meaning any overflow water cannot go into the waste pipe when its plugged.
Choosing the correct type for bathroom is important, that's why all our bath and basin wastes are clearly stated which type they are. Our customer service team is always available to answer any questions.