This is included in version 9.5.4 and up.
Activity column on purchase order lines
If you’d like to add multiple activities to a PO, then you will need to enable the correct system variable. Navigate to the Silverlight Settings and make sure that system variable 1107 is checked off.
To check if the column is active, navigate to jobs > purchase order > details. If the system variable is enabled and the column is not displayed, like illustrated below, un-hide the column. Right-click in the grid, navigate to “Hide columns,” and click the X next to “activity”.
Notice that when the activity column is displayed and you choose an activity, the activity name will be added to the subject column.
Adding an activity to PO lines also allows you to add multiple activities to the same PO.