We've recently made a minor change to the Purchase Order screen to help make it clearer when ordering interchange parts on a job. 

There's now a drop-down menu in a column titled "WO Item Sku" where you can select which item on the Work Order you want the parts you're ordering to be associated with. Note: In order for the job to be marked as sourced, you MUST tie at least one item. 

It isn't necessary to tie all items (though you should tie all those you can), but at least one must be tied. If you need to add a part to the Invoice that wasn't there previously, you can select "Add New Item to Invoice" which will create a new item on the invoice, with the associated cost, but no price to be charged on the invoice. 

What happens if I tie a PO item to one with a different SKU?
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Purchase order items that get tied to Work order items that have different SKUs will have the PO item reflected in the WO item's "Override SKU," or "Inventory SKU" field. The item SKUs on the invoice will remain unchanged, but this helps track inventory more accurately.