Absence of restriction and absence of prior identification are related but distinct. The former avoids making a feature exclusive to one individual; the latter avoids identifying a particular previously known instance.
Examples:
«زَيْدٌ كَاتِبٌ» — Writing is attributed to Zayd without making him the only writer.
«زَيْدٌ رَجُلٌ» — Zayd is described as a man, without pointing to a specific previously known man.
«زَيْدٌ الْكَاتِبُ» — Context must now determine whether the known writer is being identified or writing is being restrictively predicated.
Thus moving from indefinite to definite is not merely adding al to a word; it can change the informational structure of the assertion.