If the material is also used in the calculated design construction, Container tabs, it inherits the waste factor set in Inventory for that material. The assumption is that you will cut/modify the total material so the quantity is combined and the default waste will apply.
However, if the material added in Packing > Other is not in the Container tabs, the material is added to the Details > Materials tab on its own line item and the waste factor is set to zero so the user can enter the waste what's correct for that situation.
For Lumber added to Packing > Other, if also in the Container tabs, users can click on the DUNNAGE box and this lumber will be separated out from the total lumber and the waste factor value will be set to zero in Details > Materials so the user can enter the waste what's correct for that situation. (Lumber marked as Dunnage will appear on ISPM15 reports in the Dunnage section.)
All other materials added to Packing > Other, will appear on their own line items in Details > Materials, and are separated out from the same material in the crate construction tabs.