According to this page : https://community.bistudio.com/wiki/Arma_3_Vehicle_in_Vehicle_Transport
You can load vehicles in other vehicles if you configure everything correctly.
They will be placed automatically while there is enough space for them.

That's a great feature, but I would like to know if you could bring us the ability to add what we could call Layers.
So in 1 vehicle, you would have several zones where you can load vehicles.
The main example I can give you is this :
http://www.gardien-transports.com/gifs/porte_02.jpg
At the moment i don't see any efficient way to make a truck like this one except by using attachTo but that's not a very good idea because the positions are not the same for every vehicles ...