Warn about duplicate purchase orders numbers.
If you want to the system to warn you when creating a duplicate purchase order number, tick this checkbox.

When calculating Quantity Available for my inventory, deduct:
QuickBooks does track 2 quantities for inventory items: Quantity on Hand and Quantity Available.  Quantity on Hand increases when you purchase new inventory or receive items you ordered through a purchase order.  It decreases when you sell items by writing an invoice or entering a sales receipt, or when you make an inventory adjustment to account for theft or damage.
Quantity Available can be equal to Quantity on Hand, but you can also tell QuickBooks to deduct “Quantity reserved on Pending Builds” and “Quantity on Sales Orders” to better manage your inventory items.  By checking these boxes, QuickBooks will let you know your available quantity by deducting the existing pending builds and Sales Orders, preventing out-of-stock items sooner.

Warn if not enough inventory to sell.
If you create invoice to sell more than what you have in the inventory, QuickBooks will warn you when this checkbox is ticked.  You can still create the invoice though, but then you must order an additional inventory afterwards.
You can then select further option whether you want the warning based on the “Quantity on Hand” or “Quantity Available” by checking its button.

Unit of Measure.
With this feature enabled, you can define Single Unit of Measure or Multiple Unit of Measure per item.  Single Unit of Measure is used if you buy and sell items by the same unit of measure, for example, by piece, pound, pair, etc.
Multiple Unit of Measure is used if you buy and sell items with different unit of measure, for example, you buy by kilogram and sell it by gram.  Or, you buy by dozen and sell it by pair.  You can select the type of Unit of Measure from its count, length, weight, volume, time, area, and other:


QB Preferences Affecting Inventory 2


Once selected, click Next to define the base (smallest increment) in your unit of measure:


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You can then Add the Related Unit:


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And select default Unit of Measure:


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Finally, you can finish it by naming your unit of measure.

There is another additional preferences affecting your inventory in the Time and Expenses, on Company Preferences tab:


QB Preferences Affecting Inventory 6

When you define the Default markup Percentage and the Default Markup Account in the Invoicing Options pane, QuickBooks will default your new item Sales Price equal to the purchase price plus the percentage of the markup:


QB Preferences Affecting Inventory 7