When finishing an appointment in the Appointments book, there is an option which allows for the user to choose which services to bill, it also allows them to specify if they wish to book a follow up appointment.
To turn on this setting, from the Appointment Book screen go to Setup >> General
When the user switches the appointment to 'Finished' in the appointment book, a window will appear asking them to assign Items, Billing Rate, Notes and select a time-frame for when a Follow Up appointment should be made.
All this information will now be visible to other users at the bottom of the screen when the appointment is selected.
Example:
