Understanding load cell
Strain gauges are small precious metal conductors usually stretched to measure strain in a bridge configuration. There is good detail here: http://www.ni.com/white-paper/3642/en/   ....and here: http://www.omega.com/toc_asp/frameset.html?book=Pressure&amp;file=strain-gage-position_REF

Every strain gauge is individually calibrated since it is affected by the strain (in itself) when glued to a surface providing the measurement lever. Phidgets seem to provide extensive details on the units they manufacture. 

Since the strain gauge is an analog sensor the resolution is essentially infinite. For example if the strain gauge is glued to a thin metal strip, then the strip will deflect with very small applied forces/weight. It's only when you digitize the output you get a digital resolution implied (well explained by Phidget).