Your shipment has to be palletized or put into a liftvan. The volume your shipment will occupy in the container, is the volume of the palletized shipment. This volume is calculated by multiplying the maximum length by the maximum width by the maximum height.
The volume of a shipment placed inside a liftvan is the volume of the liftvan.

Accordingly, the volume of your shipment will be higher than the combined volume of your items. How much higher? This will depend on the nature of your shipment and how tight the items can be packed on the pallet or inside the liftvan. The allowance you choose is a rough adjustment to the combined volume of your items.