Within the client details page, you can enter notes or comments which you would like to have appear on all events you allocate a particular client to. This saves having to copy & paste it in each time you run a new event. Changes are automatically updated on all your past & upcoming events your client has booked.
Note, anyone who can view your event details will also be able to view any notes & comments you might have entered, so please keep this in mind if you are planning on storing any sensitive information here.