There are a number of options that you can use for each item that is inside the Default content tab of the combination that lets you customize exactly what happens when.
Stays in the combination when ‘Emptying combination’ in the warehouse
This option allows you to specify that the item should not be removed from the combination when emptied in the warehouse.
Reserved form stock
The 'Reserved from stock' feature helps you manage how the content of a physical combination behaves when planned on a project. You can learn more about it here.
Show on quotes - Show on packing slip
You also have the option to show the default content of a combination on documents such as quotes and packing slips.