A Message Label is an alternative way of denoting Messages between Lifelines, which is useful for 'uncluttering' Timing diagrams strewn with messages. To indicate a Message between Lifelines, draw a connector from the source Lifeline into a Message Label. Next, draw a connector from another Message Label to the target Lifeline. Note that the label names must match to reflect that the message occurs between the two Message Labels.
The following diagram illustrates how Message Labels are used to construct a message between Lifelines.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 518) states:
Labels are only notational shorthands used to prevent cluttering of the diagrams with a number of messages crisscrossing the diagram between Lifelines that are far apart. The labels denote that a Message may be disrupted by introducing labels with the same name.