In https://docs.px4.io/en/config/flight_mode.html I found out about Assist mode in the following text:
"Assist mode - This mode is added to the same channel as Stabilized/Main mode if (and only if) Position Control is enabled and defined on a different channel than Stabilized/Main."
However, QGC does not have an Assist mode. If somewhere in QGC this mode exists, then what does this mode do? If not, then we should remove this text about Assist mode.