A sequence diagram is a model describing how groups interact over
time. It is an interaction diagram that details how
operations
are carried out: what messages are sent and when.
You use the sequence diagram model to describe the aspects of your
system
that change over time. These are events that mark changes, sequences of
events
and so forth. The time sequence interaction is shown in the sequence
diagram
and consists of two dimensions. The vertical dimension (time) and the
horizontal
dimension (different objects).
UML sequence diagrams can be used to:
Select the Zoom
New Object can be created by selecting
the icon in the toolbar. Allows you to add a new instance within the
diagram. 
New Actor can be created by selecting
the icon in the toolbar
Message can be created by selecting
the icon in the toolbar. Each message represents a method call. 
Self Message can be created
by selecting the icon in the toolbar. A reflexive message is a message
sent
by an instance to itself.
Add a link between the return (lost/found
element creation) and the lifeline
Add a return (lost/found element creation)
Add a frame in the Sequence diagram and in the UML model
Add an Interaction Use
Create a Component
Create a Class
Create an Interface
Create an Entity
Create a boundary
Create a Controller
Create a Note
Create a link from the note
to any Sequence diagram element
Create a link to a file or an element 
Create a Text in the diagram (See Font and Colors option)