x1 (unitless/dimensionless number; li) is the specific gravity (density or concentration ratio) of x2 (substance) with respect to/by standard/per unit of density/concentration of x3 (substance), where density/concentration of each substance is being taken per unit x4 = nildenmi3 at locus x5 = nildenmi4 of density/concentration type x6 = nildenmi5.
See also: "nildenmi".