Custom fields enable you to customise Prospect to reflect your business' processes. With up to 40 custom fields to configure at each level of the hierarchy, the choice is yours! Some custom field-favourites from Prospect users include drop-downs, date selectors and flag fields!
Note: we'd recommend you only use custom fields if the field you're adding isn't a standard one that's already available in Prospect by default - just to avoid overcomplicating stuff.