Network keys are used to ease sync between iPADs. It means that iPADs communicate between each other using that network. It also means that two iPADs should not communicate if they have different network keys.
It is most visible when creating two full sync environments within one location. If connected on the same network, they can be separated by different network keys.
It also applies on KDSs. So if wanted to sync several POSs and several KDSs, they need to have the same network key on their settings on Management Console.
If KDSs are not receiving orders, also if expo KDS and kitchen KDS are not syncing, the same network key should be typed in both KDSs and POS.