The MASS-VIEW has actually been designed to operate in a pressure range from 0 up to 10 bar(g).
Since the MASS-VIEW series operate on the principle of direct thermal mass flow measurement, the combination of its structural design and the increased (volumetric) gas flow rate in sub-atmospheric conditions might have negative side-effects to ensure a proper measurement. 

Therefore the measurement output cannot be guaranteed in sub-atmospheric conditions.

In other words, sub-atmospheric conditions are possible, although it is limited with lower accuracy. 
Therefore, I cannot mention any lowest possible absolute pressure.