This Purchasing option in Options Configuration defaults to Y.
Yes = If you're creating a PO and you type something in the SKU field and press ENTER, the system looks for the item number entered as an SKU, then a manufacturer part number, and finally as an alternate part number.
No = If you're creating a PO and you type something in the SKU field and press ENTER, the system looks for the item entered as an SKU number only; it does not check to see if your entry is a manufacturer part number or an alternate part number. This option is useful if you have SKU numbers that are also manufacturer part numbers for other items. If you set this option to No, then to find an item by Mfg Part # or Alt Part # you must type it in and click Ctrl L or Ctrl A respectively (or in the case of mfg part # you could also type the mfg part # into the Mfg #/Spcl field and press Enter).