If you mean an event handler registered with reg-event-fx then there's no difference at all except for the value in the tracing results. Potentially, interceptors can act differently. But I don't think that's the case for any built-in interceptors.

I am using the fn-traced macro

No event shows as traced, they definitely did before but exactly when it stopped working is a mystery, I can't recall the last time I used tracing

Try following through the guide at https://github.com/day8/re-frame-debux/blob/master/README.md#option-1-installation again and see if that works.

Thanks will do and see how I go