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We present arguments based on the measured abundances in individual dampedLyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), andalso the average abundances inferred in large samples of QSO absorption linesystems, to suggest that the amount of dust in intervening QSO absorbers issmall and is not responsible for missing many QSOs in magnitude limited QSOsurveys. While we can not totally rule out a bimodal dust distribution with apopulation of very dusty, metal rich, absorbers which push the background QSOsbelow the observational threshold of current optical spectroscopic studies,based upon the current samples it appears that the metallicity in QSO absorbersdecreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thusthe sub-DLA population is more metal rich than the DLAs, a trend which maypossibly extend to the non-damped Lyman limit systems (NDLLS). Based on therecently discovered mass-metallicity relation for galaxies, we suggest thatmost sub-DLAs and possibly NDLLS, are associated with massive spiral/ellipticalgalaxies while most DLAs are associated with low mass galaxies. The sub-DLAgalaxies will then contribute a larger fraction of total mass (stellar and ISM)and therefore metals, to the cosmic budget, specially at low redshifts, ascompared to the DLAs.Comment: Expanded version. accepted for publication in A&A, 7 pages, 2 figure

The nature of damped Lyman $\mathsf{\alpha}$ and sub-damped Lyman $\mathsf{\alpha}$ absorbers