Arrival & Departure
Arrival and Departure dates specify the period of the reservation for which the customer actually pays the rent. A customer pays for a selected period with indicated Arrival and Departure dates, and the rental amount is calculated accordingly. Usually, but not always, these are exactly the same as Check-in/out dates.
Check-in & Check-out
There are cases where for example the customer leaves the apartment one day before the indicated departure date, or arrives one day later than the indicated arrival date. This influences the check-in & check-out dates which are the actual dates in which the customer physically enters and leaves the apartment. Setting these dates correctly is important for people who handle check-ins/outs so as to be aware of when to be present at the apartment for assisting the guests.
Keep this in mind when creating a reservation and make sure you specify the right check-in/out dates not only the arrival and departure dates. You can always go back to your reservation and change these dates if necessary.