Reading these figures
Plan cargo with the internal dimensions and check that every item passes through the door opening, which is smaller than the inside of the container. External dimensions matter for trucks, cranes and the space at your site.
Payload is the cargo weight the container can take: the maximum gross weight minus the container's own tare weight. Road weight limits at origin or destination can be lower still.
For what each figure means and how to choose between sizes, read Shipping Container Dimensions Explained and 20ft vs 40ft vs 40HC Container. To see how your own cartons fit, use the Container Loading Calculator.