A link is a connection path between two objects; it indicates some form of navigation and visibility between the objects is possible (see Figure 15.6). More formally, a link is an instance of an association. For example, there is a link—or path of navigation—from a Register to a Sale, along which messages may flow, such as the makePayment message.
Note that multiple messages, and messages both ways, can flow along the same single link.
Each message between objects is represented with a message expression and small arrow indicating the direction of the message. ...