SUPREME AQUA GOLD MOULDED PIPE FITTING END CAP PLAIN - SCH80 END CAP PLAIN (Size-100mm) - The two terms are frequently used interchangeably. Nevertheless, plugs press or thread inside a part, whereas caps push or thread over the part's outside. Caps and plugs keep impurities out of the opening during production, masking, storage, and shipping. They also keep out moisture and liquids.