What is the difference between the primary date variables and segment date variables?

Primary Date Variables:
@Arrival Date: Represents the guest's initial check-in date (arrival date of the first booking in a series of related bookings)
@Departure Date: Represents the guest's final check-out date (departure date of the last booking in a series of related bookings)

Segment Variables:
@Segment Start Date and @Segment End Date: Contain data for each individual booking, independent of related bookings