Order weight is calculated from the weight of all the items + weight of the package.
Item weight can be checked in the Product Details screen > Postage definition
The weight of the package can be checked in Shipping > Packaging Groups > Packaging Types
Order weight can be seen in order details (right-click on the order > View > View Order > Details)
These details can be edited manually or via import.
A composite SKU is a SKU that does not exist as a physical product but is a combination of other SKUs, either to create a "Bundle" type item or a multipack of the same SKU. The advantage to creating these items in Linnworks is that Linnworks will calculate the number of available units based on the availability of the child SKUs. Weight for composite items is also taken from the child items, therefore there is no need to add weight to a composite parent or this will cause incorrect order total weight as it will be doubled.