What does it mean "Epix: Set display on/off as a default power control and added to the existing power controls if necessary."

I can't try it yet because for me 7.24 not available. 

But: I could already put it in "light button menu" (let's call it circle menu). So what's the difference between 7.20 and 7.24 in this case? It became default there? So the only difference that in a new device it is already there and don't need to put there? (So basically for me who already put the item there, it means nothing?)

Just did a sync and GC did not display 7.24 as an update but on powering on was on the watch. Tried out Display Off in the control menu and very nice feature. With AOD and knowing you may not be looking at your watch for a while, shutting the screen down can preserve some battery life, such as when driving a few hours to do a long day adventuring. Turn on and you can quickly shut the screen off putting your palm over the watch face or hitting the upper left/control button. The opposite or a tap on the screen brings the screen back up. So, a nicer way to alternate between AOD and AOD Off than buried in the settings.

Unfortunately, if you turn off the display, gesture mode is no longer working. I wish you could toggle between AOD and gesture mode

But what's the new in it? Screen on/off is already in the circle menu for me. (7.20 "old" fw)

I never actually had used it so maybe someone can clarify what the difference is.

I see what you mean. I'm going to try AOD off this week using gestures in General Mode and use the Display On/Off if the screen coming on in the evening is bothersome. I can see in a movie theatre the screen may annoy someone next to you. I see 17 days of battery with AOD off using gestures so worth trying it out. 

** update. Tonight, I had to resort to using the Display Off, as just cooking dinner gestures were annoying. I don't need the screen turning on and off cutting and preparing food. I may do this every evening and just shut the screen down. Will give me a ton of battery life. During the day a Gesture/Glance works well but if the TV is on and you really don't need to look at a watch, I may proceed in this fashion, at least until they change functionality again. Upside is 17+ days useful life. No complaints there.

I think I preferred the original behavior of the AOD before the 7.24 update.  I.e. the AOD turned off after 60 seconds of no movement.  I know there were arguments that this meant it wasn’t an AOD but for me it was the best of both worlds, usually AOD but battery saving if I was still for longer periods of time (in the cinema, watching TV, taking a nap).

A setting for screen off after x minutes of no movement would be ideal (assuming that movement/gesture turned the AOD back on).

Fully agree, Should be configurable

I don't have 7.24 yet, but I had been thinking the same thing. I would really like more configuration options for AOD.