basis of RSD is the level of detection (LOD) and level of quantitation (LOQ), in most of all cases %RSD should be less than 10% but depending upon the method you used it can very upto 20% but not more than that in any case
%RSD is an indication of precision. It is used in every quantitative analytical technique I can think of. Reason? It is a pretty good measure of the accuracy of your result.
LOD tells you how low in concentration your instrument can qualitatively identify your analyte.
LOQ gives you the concentration limit at which your instrument can accurately measure your analyte.
In the case of plant samples your prep is your limiting factor which is why you should add an internal standard of some sort at the beginning of your sample preparation. %RSD comes into play because as I said, sample prep is the limiting factor for accuracy and precision.
Thanks Michael for your assistance in this regard. I actually wonder that how signal noise ratio (S/N) ratio is related with the %RSD? What is the relation of s/n ratio to %RSD?