外調機 is not the outdoor unit of an air conditioner but is a unit for processing the outdoor fresh air that is taken in for the sake of ventilation. Introducing fresh air from outside into the room as it is can cause additional load on the air conditioning system. To reduce such additional fresh-air load, a fresh air treatment unit or an outdoor-air processing unit is installed, which filters the air, heats or cools it by exchanging heat with the exhaust air, so that the outside air is brought to a state closer to that of the inside air before letting the outside air in.
This is clear in the meaning of this term given below in Japanese:

http://words.ecogate.jp/archives/2008/09/post_97.html

Such outside-air processing units are discussed here:

http://www.daikin.com/global_ac/products/vrv/outdoor/outline...

http://www.mitsubishi-aircon.co.uk/default.asp?url=http://ww...

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Note added at 15 hrs (2013-03-17 07:49:04 GMT)
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The first reference given above states:
外調機とは、外気負荷を処理する為の空調機のことをいいます。
空調利用時の換気の際、外気を導入しますが、そのままだと夏は暑く、冬は冷たい空気を取り込むため、空調のシステムに大きな負荷となります。
そこで、主の空調機が取り入れる前に、外気をある程度の状態まで冷暖房や加湿などの処理を行うために、外調機が用いられます。
ただし、全ての空調機に外調機が採用されているわけではありません。
From this, it is evident that 外調機 is used for bringing the outside air taken in for adding fresh air to the air indoors to a condition as close to that of the indoor air as possible. This is done either by a separate air conditioner or a heat pump or both. Having only a separate air conditioner for this purpose would be a waste of energy and also would require a large air conditioner for this purpose than the main air conditioner if the ratio of fresh air in the air blow of the indoor fan or duct to the recycled indoor air is high. Hence, a heat exchanger is preferred, but an additional air conditioner is used for processing the outdoor air if the heat exchanger is not capable of reducing the load on the main air conditioner sufficiently.
Not all air conditioners have an outdoor-air processing unit.
Also, not all outdoor-air processing units have an air conditioner.
Translating a 外調機 as an outdoor unit does not convey its purpose of processing the outdoor air and also may imply that all 屋外機

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Note added at 15 hrs (2013-03-17 07:49:56 GMT)
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outdoor units have the function of outdoor-air processing.