Lifeline
A lifeline is a box or a stick figure at the top, with a vertical dashed line below it. Declaration order sets the left-to-right order, and a name must be declared before use.
diagram sequence "Heads"
actor Operator as op
participant "Controller" as ctl <<service>>
op -> ctl : mount()
Syntax
participant <name> [as <alias>] [<<stereotype>>]
actor <name> [as <alias>] [<<stereotype>>]
Heads stay at the top and are not repeated at the bottom.
Specification
UML 2.5.1, 17.3.4.1 (p. 572). The head is a rectangle, or the classifier’s shape; a stick figure for an actor.