Nibal,

The Critical Station is just the common name of the valve with the highest pressure demands. There is not a common term for the valve with the highest flow demands, it is simply "the one with highest flow." Since the determination of the highest pressure demands requires intense calculations, the significance has lent to noting the worst-case valve, as the Critical Station.
Note that you can also generate a Valve Schedule with all relevant data, to see the exact demands of every valve.
This question is certainly more of a general Irrigation design theory than the operation of the software. I would recommend any of the instructional books and content provided by the Irrigation.org

--J