Bottom Drain / Suction Plug is used in most Lifegard All in One style aquariums. This is used to cover the bottom hole that is provided in the back wall for multi layer circulation. You can use a 1/2" Low profile strainer is you want water to flow from the bottom as well as the top. Or use this plug to plug off the flow.
If going on vacation it is recommended to remove the plus and replace with strainer so the water evaporation will not drop the level below the top drain and make the pump run dry that can damage it. By doing so you will increase the water top-off duration.