Lifeline
The lifeline 
element is an individual participant in an interaction. UModel also allows you to insert other elements into the sequence diagram, e.g. classes and actors. Each of these elements appear as a new lifeline once they have been dragged into the diagram pane from the Model Tree tab.
The "lifeline" label appears in a bar at the top of the sequence diagram. Labels can be repositioned and resized in the bar, with changes taking immediate effect in the diagram tab. You can also redefine the label colors/gradient using the "Header Gradient" combo boxes in the Styles tab.
To create a multiline lifeline, press Ctrl+Enter to create a new line.
Most classifier types can be inserted into the sequence diagram. The "represents" field in the Properties tab displays the element type that is acting as the lifeline. Dragging typed properties onto a sequence diagram also creates a lifeline.
Execution Specification (Object activation)
An execution specification (activation) is displayed as a box (rectangle) on the object lifeline. An activation is the execution of a procedure and the time needed for any nested procedures to execute. Activation boxes are automatically created when a message is created between two lifelines.
A recursive, or self message (one that calls a different method in the same class) creates stacked activation boxes.
To display/hide activation boxes:
•Click the Styles tab and scroll to the bottom of the list.
The "Show Execution Specifications" combo box allows you to show/hide the activation boxes in the sequence diagram.
Lifeline attributes
The destruction check box allows you to add a destruction marker, or stop, to the lifeline without having to use a destruction message.
The selector field allows you to enter an expression that specifies the particular part represented by the lifeline, if the ConnectableElement is multivalued, i.e. has a multiplicity greater than one.
Goto lifeline element
Right clicking a lifeline allows you to select Goto XXX, where XXX is the specific lifeline type that you clicked. The element will then be visible in the Model Tree window.