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The main categories of licence issued are all type rated and are only valid when also authorised by a Part 145 Maintenance Organisation. The principle is that they confer CRS certification approval when valid. Work actually carried our prior to such certification may be done by others without such licences working within an approved organisation and under its approved system. Category B1 permits CRS certification in respect of airframe structures, powerplants and mechanically-based electrical systems. Category B2 permits CRS certification in respect of avionics and avionics-based electrical systems. Category C applies to an aircraft in its entirety in respect of CRS certification in base maintenance provided that such an aircraft is wholly within a Part 145 organisation. The system of engineer licensing used in the USA by the FAA is entirely different and not based upon aircraft type ratings.