Arrows are used to graphically represent messages that are passed between objects. The type of arrow indicates the type of message that is being passed:
Synchronous messages are shown as a solid line with a solid arrowhead. A synchronous message is one in which the sender must wait for a response before it can continue. Asynchronous messages are represented by a solid line with a lined arrowhead. With an asynchronous message, the sender does not have to wait for a response before continuing. Reply/Return messages and asynchronous return messages are both represented by a dashed line with a lined arrowhead.
