The second principal purpose is nawʿiyyah (kind or type): drawing attention to a distinctive kind or quality rather than merely selecting one unknown member of a genus.
وَعَلَى أَبْصَارِهِمْ غِشَاوَةٌ
And over their eyes is a covering. (Al-Baqarah 2:7)
«غِشَاوَةٌ» is indefinite. Read as indicating type, it evokes a particular quality of veiling rather than simply asserting the presence of any randomly selected cover. Individual indefiniteness and qualitative specification are different semantic possibilities.