BTW what is "24p mode on" exactly?
Home screen - Settings - Playback - 24Hz.
It should be on for frame rate matching video fps.
Needs to be relabelled really because it also enables 29.970 switching.
So the answer to my question is something like: It's an option to enable 23.976, 24, ...?, ...?, 29.970 FPS playback with matching frequency (Hz) to the device's output. Without this setting the video output frequency will always be the one set in the Home screen - Settings - Display menu.
Do I understand this correctly?