How do you decide between filling by volume or by weight?

Check which limit your goods reach first.

For light, bulky products, such as packaging, textiles or furniture, the container will be full by volume long before it approaches its payload limit, so cubic metres are the constraint to plan against. For dense products, such as liquids, ceramics or metal parts, the payload limit is reached while much of the space is still empty, so weight becomes the constraint.

Many importers handle both kinds of supplier, which means the constraint should be set per supplier or per product family rather than once for the whole business. A planning system should check both limits on every order and flag whichever one binds, rather than assuming volume is always the answer.