Notification cards are the blocks that are not visible to the clients but will send you the notifications via email, sms or push.
To add the card you need to click gren/red icon in the service blocks section:
Then you add the card anywhere in the conversation flow. You can choose: White/Green/Red level of urgency, agent to be notified, email and/or phone to receive the SMS notification.
Please note that White cards are free but only have email channel, not push or SMS.