Put simply, the performance engineers are allowed to include some clearway when calculating the performance figures. As clearway is not necesarily load bearing, it cannot be used for Acelerate Stop distances but it can be used for the go case.
Therefore the V1 can be calculated to allow the aircraft to accelerate on all engines to V1 and then stop by the end of the runway/stopway, or to continue from V1 on 1 engine and achieve 35' screen height by the end of the clearway, which may be further than the end of the stop way. There is a maximum amount of clearway that is allowed. From memory it is up to a maximum of 50% of the stopway. But don't quote me on that.
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The Rev