After giving the boot capacity, they then refer to the passenger space within the car.
The interior of the car is the "habitáculo" and "cota de habitabilidad" is the space widthwise and lengthwise from the pedals to the backseat.



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Note added at 56 mins (2006-02-24 14:14:36 GMT) Post-grading
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As you can see from the following ref., it refers to the space occupied by the passengers.

Nada se puede achacar a la postura al volante; los asientos son cómodos y agarran bastante bien en sus plazas delanteras. En las plazas traseras, la cota de habitabilidad es algo justa para tres personas, tanto en anchura como en altura.
http://localizador.racc.es/index.racc/mod.actualidadHome/mem...

The following gives the definition above, i.e. the space between the pedals and the backset, lengthwise; and the width, obviously