The trigger is set to fire when the signal enters the top window, but the signal never enters the top window. The legend for the trigger (upper left) is actually clearer than the text.
I'm not sure there's anything clear about Rigol's legends and names for this (and some other) triggers
They also seem to have skimped on storage space for the different trigger icons used at the top, which rarely change due to conditions you set for the trigger (they only change for Edge, Pulse, and Slope).
The icon you posted stays the same regardless of conditions. And the delta icon and level doesn't make sense if you think of the area as 2 windows. They only use it because they don't devote enough screen real estate for two distinct voltage levels.
I wrote software for displaying extended trigger information in RUU because, for the complex triggers, the amount of info Rigol displays is pathetic. Take a look at one of my screen images posted from RUU (with Trigger Level indicators turned ON) and decide if it or the DSO provides better insight into what the Windows trigger is doing