Top and bottom guidance refer to the mechanisms that guide the movement of a sliding door. Top guidance is a component mounted at the top of the door and helps to keep the door aligned as it moves along the tracks. Bottom guidance, or bottom roller, is a component mounted at the bottom of the door and helps to keep the door stable as it moves. Both the top guidance and the bottom guidance help to ensure that the door moves smoothly and evenly along the tracks, and they also help to prevent the door from swaying or swinging when opening or closing. In a lift-and-slide door, the top and bottom guidance help to guide the door as it is lifted and opened.