But just setting this will not display the custom field on activity related lists of other standard objects.
To display on activities of related standard object, click on page layout of standard object,
Example: Account Object
Click on Related Lists and settings,
Select the new custom field from “Available Fields” list and move to Selected Fields.
Save the changes and you will now be able to view the custom field in the activities of standard object.
Written by Prabha Krishnamurthy