In sequence diagrams, you can move a lifeline to reposition it horizontally in an interaction frame.
Although the horizontal position of a lifeline does not have semantic value in the model, you can move a lifeline to a location that makes the diagram easier to read. For example, you typically position the source lifeline, from which the first message originates, on the left side of the diagram.
You cannot move a lifeline vertically. A lifeline without an incoming create message is aligned toward the top of the interaction frame and a lifeline with an incoming create message is aligned at the level of the create message. You cannot move a lifeline on top of another lifeline in an interaction frame.
To move a lifeline:
Parent topic: Creating lifelines in sequence diagrams
Related concepts
Lifelines