The alert note is a special type of Ticket comment. Once it's filled, a red exclamation mark will appear on the appointment card. Hover your mouse on it, you will quickly see the note.
In the old version, it's called Notice.

Add alert notes upon creating a new appointment
- At the appointment booking page, input alert notes under the alert notes section.
- Click <Booking now>, an appointment will be created along with notes

Add alert notes to existing/finished appointments
- Go to <Appointments>
- Click an existing/finished appointment
- Click <Add alert notes>

Further read: