The "state" property can be

Booking States:

accepted (participant accepted booking message)
declined (participant declined booking message)

Confirmation States:

confirmed (participant confirmed)
cancelled (participant cancelled)
not_coming (participant cancelled)
needs_reschedule (participant cancelled and requested a reschedule)

Check In States:

noshow (participant was a noshow)
arrived (participant arrived)
ontime (participant arrived ontime)
late (participant arrived late)

Example Result:

{
  id: 123,
  calendar_id: 456,
  start_time: "2016-04-15T12:00:00-07:00",
  end_time: "2016-04-15T13:00:00-07:00",
  title: "Appointment with Fred",
  location: "",
  content: "",
  is_deleted: false,
  participants: [ {
    id: 789,
    name: "Fred Smith",
    phone: "650-555-1212",
    normalized_phone: "+16505551212",
    email: "fred@gmail.com",
    state: "initial", 
  }],
  custom_data: {
    field_1: "custom value"
  }
}