For the case of optically thin lines, we show that the relation existing between the first order moment of a P Cygni line profile and the quantity n (level) (cf. Castor, Lutz and Seaton, 1981), where is the mass-loss rate of the central star and n (level) the fractional abundance of the relevant ion, is in fact independent of any Sobolev-type approximations used for the transfer of line radiation. Consequently, all results established in the context of “very rapidly” expanding atmospheres (see Surdej, 1982) and mainly referring to the non-dependence of W1 on various physical (underlying photospheric absorption line, limb darkening, collisions, multiplet line transitions, etc.) and geometrical (radial and rotational velocity fields, size of the atmosphere, etc) effects remain unchanged for arbitrary (e.g. non-Sobolev type) outward-accelerating velocity laws.