The service determines the "reason" for the appointment. If you are a Massage Therapist, your service could be called "Hot Stone Massage" and thus when the appointment is made the client sees that he/she is booked for a Hot Stone Massage and will choose between the lengths of massages you offer. If you are a Tutor, your service might be called "One on One Tutoring Session" and, similarly, when your students book an appointment they will choose that from your list of services as the reason for their appointment.
Most of the time, service appointments are made with a client who will receive email notifications about the appointment. You can make appointments using the Back Office of TimeTap and not attach a client to it, if you want to.
Class Appointments: When you add a class to your account, you also add a schedule for when sessions of that class will be held. This is different than services as service appointment times are offered based on a breakdown of your staff's "Service Availability". Once you've created a session for a class, that session will mark you as "Busy" and prevent clients from being able to book service based appointments with you.
The class will also determine the reason for the appointment. If you are a yoga instructor, you may have several different types of classes (60 Minute Vinyasa Flow, 75 Minute Vinyasa Flow, Balance, etc.) and your clients could choose between all of your classes to see what your schedule is like for each.
One class session may be comprised of several different appointments, depending on the number of clients that registered for the class session. The total number of possible appointments that could be made for a single class session is determined by the class session's max capacity.