You can freely enter the expected arrival date. In that case, it is that date that will be taken into account. It cannot be recalculated.
You can also leave the field empty: this date will then be calculated automatically at the time of creation based on the departure date. The expected arrival date is calculated as follows:
Arrival date + Transport lead time between the departure place and arrival place + release lead time at arrival place + potential total days difference.
The transport lead time considered between the departure place and arrival place is the one defined for the specified transportation mode and carrier. If this setup does not exist for the given carrier, the transport lead time defined for the transportation mode is considered. If no setup has been defined for this transportation mode, then the transport lead time defined between the departure place and arrival place is considered. Otherwise, the calculation is performed with no transport lead time.
When using the transport with a shipment, this date populates the expected arrival date of the shipment.