participant Name [as Label]

A->B: text  A-->B: text  A->>B: async  A-->>B: async reply

note over/left of/right of A[,B]: text

activate A  deactivate A

loop / alt / opt / par label ... else ... end

autonumber [on|off]  — numbers arrows in a red circle from that point on

Drag any participant box left/right on the canvas to reposition it — use Reset layout to undo.