It sounds like you have 2 of our instruments confused, the LP-80 is our linear ceptometer that measures leaf area index. It sounds like you are talking about the leaf porometer here, so that's what I will answer. mmol/m2s (or mol/m2s or micromol/m2s) are common units for a flux density (rate for a given area) in plant physiology. These units are very convenient because you can express flux densities for CO2, O2, H20, and even light in these same units and calculate an efficiency (such as water use efficiency or quantum efficiency). This is the primary reason to express this value as stomatal conductance rather than stomatal resistance.

Stomatal conductance differs from transpiration in that it is independent of the water vapor gradient between leaf and atmosphere. Just because you have a stomatal conductance of 200 mmol/m2s, does not mean the plant is losing (transpiration) 200 mmol/m2s of water through the stomates. The transpiration rate depends on how open the stomates are (stomatal conductance) and the water vapor gradient between leaf and atmosphere (vapor pressure deficit or VPD). There are other considerations such as boundary layer conductance that can effect transpiration but for which stomatal conductance calculations are independent.