If Air has this new thing, check in time from ’ ’ until such and such ’ '. then is this a rule break if they turn up later, without notice and agreement at booking?
Technically, yes. It sounds like you have something in mind, though. What’s your reason for asking?
The usual! yes and ive done this and that…they know everything! lol
We have a saying-if you didnt laugh, you’ll cry’ 
I have those guests all the time. I have a lot of guests who just turn up at noon and ask me to let them in. Check in time is 2pm or later. If the place is already clean and everything is ready, I make an exception. But if I don’t feel like letting them in or I still need to finish cleaning, I tell them to wait until 2pm. Unfortunately many guests don’t read the rules. I even wrote it in my rules, that they can not check in before 2pm. Waste of printed letters.