OPEN = the Sales Order is still being processed.
FORCE CLOSED = the Sales Order has not been delivered in full, but no further deliveries will be made
You might have a Sales Order that has not been delivered or only partially delivered to the customer, but the balance will no longer be processed. In this case you need to FORCE CLOSE this order. This will help you tracking that this order no longer requires further delivery notes.
For INVENTORY USERS: this is also the way you tell Zedonk that those undelivered units are now free and they will show in your AVAILABLE TO SELL and your PROJECTED STOCK.