Easiest to explain by contrasting behaviors of scopes with or without 'vertical mode' triggering.
With: The scope has two analog input channels, both initially active. Set up trigger parameters (slope, coupling, etc.) on whichever. Enable Vertical channel trigger mode. If either channel is stopped, or probe removed, triggering automatically switches to the input channel still running, with the same trigger parameters (I think).
Withouth (Siglent). With 2 operating channels, set up trigger on either one. Without vertical mode, if the channel selected as trigger input is stopped, disconnected, etc., the operating channel continues looking for trigger on the non-operational channel, and fails. To re-establish trigger on the still-operating channel, one 'rows the UI' to selected that channel for the trigger input.
On the Sigalent, some operational modes (can't recite which, off the top), apparently default to trigger from channel 1 even though only channel 2 is running.
You could row the UI a different way, to hide one channel rather than turn it off, and the scope would continue to display the other channel as before. But if the non-operational channel probe is removed to, say, select a different circuit node, the operating channel likely ceases to trigger.
Have you ever used a scope with 'vertical channel' trigger mode?
-Fabrice
Experience is what you get just after you need it.