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The list of statuses can be extended if the Custom Customers Orders Statuses option is selected on the Administration > Settings > Sales form. For details, refer to Settings: Sales.
The Reserve Inventory button becomes enabled once the order is posted and its status is changed to In Process. Click this button to create a reservation document which will indicate that specific items for this customer order should be reserved in a specific warehouse or warehouses. For details, refer to Reserving the Inventory.
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The Document Settings dialog box