An indefinite noun refers to an unspecified member of a class. Adding a modifier can narrow that class without determining a unique individual.
«رَجُلٌ جَاءَ» — a man came, with no stated qualification.
«رَجُلٌ صَالِحٌ جَاءَ» — a righteous man came; the class is narrowed.
«رَجُلٌ صَالِحٌ عَالِمٌ جَاءَ» — a righteous scholar came; further characteristics are supplied.
Traditional grammar often calls this effect takhṣīṣ (restriction). Restricting a range does not necessarily establish full personal identity; restriction and unique identification are distinct.