A lifeline is a named element in a sequence diagram template representing an individual participant. So, in a sequence diagram template , each instance is represented by a lifeline. A sequence diagram's lifeline elements are at the top. A sequence diagram is an interaction diagram that depicts objects as lifelines running down the page, with messages drawn as arrows from the source lifeline to the target lifeline to represent their interactions over time. Sequence diagrams are useful for displaying which objects communicate with which other things and what messages initiate those communications. Sequence diagrams are not meant to depict complex procedural logic
