Superside is a word that's not often used in modern English. It is mainly referred to a side of a car that is usually used for placing advertisements. However, there are alternative words that could be used instead of superside, depending on the context of use. Words like panel, section, segment, and flank could be used as substitutes for superside. These words also represent a part or section of a whole. In the case of advertising on a car, it is appropriate to refer to the side of the car as the advertising panel, segment, section or flank as well as its superside. Hence, there are a wide number of words one can use in place of superside.