What is used to indicate flow direction in a flow diagram?

In a flow diagram, arrowheads are specifically used to indicate the direction of flow. This visual representation allows the reader to easily understand the movement of fluids or materials through the system being depicted. The shape of arrowheads not only provides clarity but also minimizes confusion, as they clearly point from one element to another, signifying the path along which the flow occurs.

The other options, while they may seem relevant in different contexts, do not serve the specific function of indicating flow direction as effectively as arrowheads do in technical diagrams. Signs and markers can denote features or conditions but do not inherently convey movement. Points may represent specific locations in a system, but they do not indicate flow direction without additional context. Thus, the use of arrowheads is essential for accurately communicating the dynamics of a system in flow diagrams.