The “Schedule out of stock Products” option is used to stop Acctivate from automatically scheduling the line. If the item isn’t automatically scheduled, invoicing the order will skip the line item in question and only include the scheduled lines. The remaining unscheduled lines will remain backordered and the order will then have a status of ‘Backordered’. Disabling the “Schedule out of stock Products” Sales Order option is recommended in order to avoid selling product into the negative.
Another option you could review is a User Permission option to block the overriding of the scheduled quantity on the Sales Order. You can find this in Configuration Manager -> User Information -> OrderProcessing -> Override Scheduled Qty. When this permission is set to “Yes”, users can type in any value for the Scheduled quantity on a Sales Order (so long as it doesn’t exceed the Ordered Quantity for that line item). When set to “No”, users cannot override the Scheduled quantity on a Sales Order, and only in stock products can be sold.