Contactors can be lower than 5kW, if you look at the construction difference you can see that relays usually have open contacts where you can see them but contactors do not. This is generally because contactors are designed to control the arc upon opening and an arc will result from higher current levels and different loads. In my experience the description is true regarding relays for 'control circuits' and contactors for 'power circuits' although like everything there will be exceptions such as a control circuit for loads like very large contactor coils.