The basin waste, there are two types, slotted and unslotted. If your basin has an overflow, you will require a slotted basin waste and, if not, an unslotted one.

Slotted Basin Waste
Far and away the most common type of waste, a slotted waste is suitable for use with a basin that has an overflow. The slot in the waste allows any excess water from the overflow to drain into the waste pipe when the plug is closed. Basins with an overflow hole must have a pop-up waste with an overflow.

Unslotted Basin Waste
Unslotted basin waste is used in sink without overflow hole. They offer a clean, streamlined appearance, making them a favorite in minimalist bathroom designs.