Complex sentence: The term refers to the fact that the tongues for each pipe are not enclosed in their own boot, as in standard reed pipes, rather all the tongues are held together in the same chamber, much like in a harmonica or an accordion.
Split into simpler sentences: The term refers to the absence of enclosure around the tongues for each pipe: they are not enclosed in their own boot, as in normal reed pipes. All the tongues are held together in the same chamber, as in a harmonica or an accordion.