Notifications and Notices are both important parts of getting information to your subscribers, but they do different things.
Here's a quick guide to the difference between a notification and a notice:
Notification: This is an alert that's sent to a user or subscriber from the platform in the form of a push notification, an email or SMS (text). Notifications can be managed by the individual user, per channel on the channel settings page of the mobile app and member's view of the web app
Notice: In the content center, it's possible to cret posts and notices. A notice, is a short, text-only message that can be sent out from a channel, used typically to alert subscribers to timely information, urgent messages, or key building updates. Anothe important feature of notices is that they are able to send a notification to every targeted recipient, regardless of their notification settings for that channel.
Sending a notice with a notification is therefore reserved for highly important or very timely information.