Items with serial numbers
Add New Item
Go to Inventory-Equipment Settings-Items logical categories
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Image: Items logical categories
Click ADD NEW ITEM button
Enter new Item's data and check TRACK ITEM BY SIZES radio button
Click SAVE button
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Image: Add new Item
Select item from list to: Check Details, Edit Item, Enter Quantities...
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Image: Item Details
Edit Item and click SAVE button
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Image: Edit Item
Enter Quantity for each size and click SAVE button
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Image: Enter Quantities