Event Occurrence (General Value)   An event occurrence is both a type of interaction fragment and a type of message end. Event occurrences mark the points on a lifeline where events occur. An event occurrence is the basic unit of meaning in an interaction. A lifeline is used to describe the way an instance, or a classifier playing a role, changes over time and event occurrences mark the points on lifelines where messages start or end.

Explanation

An interaction typically involves two or more lifelines. There are significant points on these lifelines called event occurrences where the instance represented by the lifelines sends or receives a message or begins or ends its existence with respect to the diagram. An event occurrence can be seen on an interaction diagram where the line changes in some way; the way it changes depends on the type of diagram. Event occurrences are the basic unit of meaning in an interaction and are ordered along the continuum of the lifeline. This concept occurs in our daily lives when we mark events such as birthdays and other important dates on a calendar.