If they are truly professional movers, and Not Casual Labour (bums picked up from the street a few blocks away, aka day labour), they Know from Experience (and law suits).
[*] which fragile item need to be expertly protected when packed.
[*] How much weight a container can be loaded with, and still be comfortably picked up.
[*] What may be stacked without crushing.
[Tip] READ the SHIPPING Contract., before you hire.. :
[Tip] use the drawers in movable furniture as packing "boxes". Otherwise, you are paying for the Shipping of Air.
[Tip] Carry Valuables in your vehicle, with you.
[Tip] Make a list of what is shipping, and take photographs of items.
[Tip] Record serial numbers.
[Tip] Inspect the items, Make a note of any imperfections/damage so you will know if at the other end you Think the movers did it. This saves every ones senses when delivered.