Terminators (terminate symbols) in sequence diagrams snap onto the ends
of activations and lifelines. Once snapped into place, they remain
attached when the activation or lifeline moves or is copied. If the
activation or lifeline symbol is deleted, the attached terminate symbol
is deleted as well. Simply select a terminator from the style bar, drag
it near the end of an activation or lifeline, and let it snap into place.
(State symbols can also be snapped to lifelines in this manner.)