In this case, the dependency indcates that if the DataInput interface changes, the OrderReader may also have to change. Frozen indicates that the value of that attribute . . . may not change during the lifetime of the source object. . . . Read-only implies that a value cannot be changed directly but may change due to a change in some other value. For instance, if a person has a date of birth an an age, the age may be read-only, but it cannot be frozen. In sequence diagrams, the activation box begins at the arrowhead of the synchronous message arrow.
Time flows strictly from the top of the page downward.
Activation at the arrowhead of a synchronous message indicates passing control to that particular function, so that it begins to execute. Hence the activation box begins there. Indicate its what it means. Each message is represented by an arrow between the lifelines of two objects. The half-arrowheads indicate an asynchronous message.