For the uninformed a cylinder refer's to a vessal that holds a gas. A tank holds a liquified gas, size matter's not , just because it's small doesn't make it so.
NH3, is a gas at atmospheric pressure and a liquid under pressure, you would usually only find a vessel of the size he was referring to ~(160 lbs) of NH3 filled in a cylinder, in the gaseous state. NH3 is usually transported in bulk under pressure, ie. in a tank. Suppose you have a cylinder of gaseous NH3 that is heated and the pressure builds up and the NH3 liquefies, is the vessel no longer considered a cylinder, is it now considered a tank? The fact is that quite often you may have a cylinder of NH3 that contains liquid NH3, this is why a cylinder should never be filled more than 85% of the capacity by tare weight as marked on the cylinder by the manufacturer, thus allowing for expansion.