If I understand that the main work of n 416 Repair and Salvage Unit were to prepared and issued Boston & Mitchell to operational units, aircraft repaired, intruments, electronical, signals, armament components but also engines and power plants prepared and issued or engines stripped and despatched, what was the main job of n511 Forward Repair Unit ?

What were the rules to send a Boston to 416 RSU rather than 511 FRU ?

Thanks in advance