The setting Show Distributed Purchase Order in italics (takes longer) is only available if the setting Linked requirement/supply planning for P-part (Lot-for-lot) has been activated, under Material Requirements Planning under the Inventory tab in the Settings procedure.
If the setting is activated, a check is made during clearance to see if there are several purchase orders linked to the same material. If so, this will be shown in the list with the purchase order number in italics on a material row. By placing the cursor over the purchase order number field in the list you will see a tooltip regarding which order numbers that are linked to the material row.
This setting is not activated by default because the check to see if there are several purchase orders linked to the same material row, makes the clearance take longer to perform.