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If you're adjusting just one of a couple of items, you can do it directly in by accessing Inventory → Supplies and open the supply details.
Add the multiple suppliers in the Suppliers tabClick on the Suppliers tab and add the other suppliers from there.
In the example below, our default supplier is A & G Engineering and we added two more suppliers for the same item.
Doing it this way allows you to purchase items from different suppliers and items still get allocated to the PO and Job considering that there is one common identifier which is the manufacturer code that we enter in the Part Number field.