When creating an appointment of any kind, you can add event notes which will show up on the appointment when you hover and the appointment when you open it.
Here are where you can add these notes in a Market Appraisal appointment:
This is where they show on the appointment page:
On the calendar, for all appointment types, you will see this when you hover on desktops and when you open the appointment:
For viewings, Notes are event notes! Anything you add in notes will show up just like any other event note.