A butt plug must be cleaned thoroughly after every use, and that's where the IP rating makes the biggest difference. Hush 2 and Bullseye are IPX7 according to the manufacturers, which under the IEC 60529 standard means they can withstand immersion in one metre of water for 30 minutes. They can be washed under the tap with soap and water.
Others have no protection. In the customer information for Cuties, ORION states that the unit must not be submerged in water, even though two shops list IPX5 and IPX7 respectively. Black Velvets Small must not be dipped in water either, and the battery compartment must be kept dry. According to the manufacturer, Anal Pusher must not be submerged. Plugs like these are cleaned with warm water and mild soap on a cloth, without holding them under running water.
Rebel is a cautionary example. The packaging says IPX7, but the manufacturer's own user manual says the device must not be dipped in water. When the information is contradictory: follow the more cautious one. A removable motor, as on Black Velvets Small, is an advantage because the silicone body can then be washed separately. More on the ratings in waterproof sex toys, and on cleaning in how to clean sex toys.
Never boil a plug with a motor or battery, and don't put it in the dishwasher – not even if it's IPX7. The IP rating is about water, not heat.