In FCL the customer books the whole container, packs it at its own warehouse and seals it. Freight is charged per container, full or half empty, and the shipping line never sees the goods: the bill of lading will say “said to contain”.
It pays off when the volume approaches the container’s capacity or when the goods are sensitive and should not share space. The shipper is responsible for the internal packing and for declaring the VGM before loading.