Maybe the aircraft size J41 or smaller is due to Citywing having their own 4x4 de-icing rig which was used for the LET & D328 aircraft in previous years. I suppose this could also be used for the J31 of Links Air and any J41.
The airport or any airline could request for the service if they are desperate and willing to pay the Citywing fee.
(This is obviously based on the assumption that the rig got transfered over to Citywing from Manx2.com)