Execution Occurrence   An execution occurrence (activation) represents a period in the participant 's lifetime when it is performing an action or waiting for another participant to complete an action and return. A lifeline can have a number of execution occurrences that and these will be positioned along the lifeline, obscuring the dashed line.

Explanation

The thin rectangle represents two related but different things. It represents the period in which the participant is engaged in an action, and its relative position on the stack. The action may be its own execution or it may have made calls to other participants. In this way, it represents both the duration of the action, and also its relative location on the stack with respect to its caller.

Related Entries

Message (Instance Creation)  Used Together The instance creation is related to the activation as elements that are used together. When an instance is created it immediately gets a lifeline. When it is called into action the activation represents this engagement.