Use Alternative Purchase Part Base Data page to define which purchase parts are interchangeable with each other. You can either enter lines manually or copy part records from the part catalog.
The Alternate Part No list of value shows all parts that have been entered as alternates on the Alternate Parts tab on the Part page. However, it's still possible to manually enter a part not included in the list. This generates a question asking you whether or not you want proceed by automatically create the corresponding record on Alternative Parts tab on the Part page.
It's also possible to use Copy Alternate Parts. In the dialog, enter the part number you want to copy from the part catalog. The records will appear on Alternative Purchase Part Base Data page, assuming the record does not exist already.
The parts must have been defined as purchase parts at the same site.
The Alternate Parts Exist option will automatically be enabled in the purchase part record for the purchase part.
Once the purchase part has been entered on a purchase order line, Add/Change Alternate Parts becomes available.