But I don't understand how it is triggered when reloading.

An additional trigger is needed, isn't it?

What does on_display make? What's the difference with a "normal" advice thread?

Sorry, maybe I've missed something.

I used the system "campaign script -> background script" to launch it at the start of a campaign (this is not new for me), but I had to use a normal trigger (click on settlement for example) when reloading. I'd like to understand the difference.