Onto and into are used when there is a transition. If something is off A and moves or is moved so that it is on A, then it has moved onto A. "He put the luggage onto the rack." Note that many times, we say "on the rack". This would be correct and accepted. It doesn't work in the other direction. If something is already resting on the table and there was no movement to get it there, you would not say "The book is onto the table." 'On' and 'onto' have the same relationship. I can go in the house or into the house. If I am already inside the house, I can say that I am in it, but not into it.