In a workflow diagram, the user interface symbol represents the point at which a user interacts with a system or application. It indicates the point where a user inputs data or makes a selection, and where the system displays information or results back to the user. The user interface symbol is typically represented by a rectangle with rounded corners, and may include labels to indicate the specific type of interface, such as a button or text box. It is an important symbol in workflow diagrams as it helps to identify the different touchpoints where users interact with the system, and can be used to guide the design of the user interface for optimal usability and efficiency.