There’s an asynchronous notation in sequence diagrams if responses can go back in different order. Still uses horizontal arrows but the arrowhead is different.

There’s an asynchronous notation in sequence diagrams if responses can go back in different order. Still uses horizontal arrows but the arrowhead is different. I don’t think diagonal arrows really help with this and will be harder to deal with because they could get overlapping really quickly. But if things are truly out of sequence use something other than a sequence diagram. Just a regular flow chart, or activity diagram, or state diagram.