Upvote:2
Very few items are to be removed from your carry on and put into the trays. At the moment it's laptops and tablets. At some screening points it's the little bag of liquids. Everything else, they don't care where you put it but they care about time. If you have, despite all the signs, video screens, and behaviours of the people in front of you in line, reached the front of the line with your pockets full of keys, coins, a phone, a wallet, and so on, the quickest thing for you to do with those is toss them into the tray. But if you move all those into your carry on before you get to the front of the line, nobody is going to care.
And yes, a carry on bag can become temporarily overweight or overstuffed because you put things into it before security, and nobody will care as long as it's back within limits when you actually board the plane, having put that stuff back into your pockets. It's all ok.
Upvote:5
The purpose is to simplify things but it also slows down scanning. In cases where there is a huge line of people accumulating at security, you may even be asked to put everything in your carry on. This has happened to me at LGA (New York) last year.
You are free to put everything in any of your bags or even in pockets of a jacket, although I would only recommend that if your jacket has zippered pockets. Then all your stuff goes into the jacket and into the X-ray machine as one item. This year I have been travelling with a vest and find it life-changing!
As you said something need to be separated in order to allow the X-ray be easier to recognize. This is required for laptops at least. Depending on the airport, they may ask it for tablets, ultrabooks and even netbooks.
The two pieces of luggage are a carry-on and a personal item. Most but not all airlines allow both but check before traveling. You can put pretty much anything that is not required to be on the tray in either of them. From the point-of-view of security, there is no difference between them.
Usually you and your items pass more-or-less at the same time. You are asked to stay with your bag until it enters into the X-ray machine and wait for it to come out once you are done your scan. This is to make sure someone is responsible for the luggage and keeping an eye on it at all times.