Sandox mode allows you to testing your notification and outright experimentation before sending it to actual subscribers. If notification is marked as sandbox, those notification won't send to subscribers however, it will gather detail about how many subscribers receive that notification. For sandbox notification, we calculate notification clicked on assumption basis.
To enabled notification sandbox mode, toggle sandbox mode switch inside Name section on create notification page.
Once your notification is live, you won't able to enable sandbox mode to that notification.