A country like Serbia obligates you to register your stay within 24h. Now how to understand that if you move constantly and do not stay in a place where they do it for you.
Do you have to register every 24h in a place you actually are? Do you have to register only once after crossing the border? Or you do not have to register at all, as you do not stay more than 24h in one place? If you stay less than 24h in that country (transit e.g.) do you still need to register?
How does the registration look in practice? Do you have to specify how long are you staying? What to do if that changes (shorter or longer)?
Do airlines (or other firms beside hotels, campings, etc.) do the registration for you?
And if you stay in a hotel, should you always get a confirmation of your registration? Does it have to be an official paper or a receipt from the hotel suffices?