Apple TV 4K
Should I use HDMI signal plus on 4K tv for Apple TV 4K?
Samsung’s “Input Signal Plus” is said to expand the ‘range’ for the selected HDMI port. Intended for gaming (computers or consoles like Xbox and PlayStation), Samsung is not very forthcoming about what it does precisely, or any non-gaming uses. How to get the best settings for UHD gaming on your TV - Samsung Support. There is some speculation on avforums.com and similar, but nothing I saw there was authoritative in my eyes.
It is compatible with Apple TV 4K, as users have this active and can use all the functionality without apparent downsides. Perhaps not all HDMI devices are compatible with this setting, hence the option to switch it on or off?