«وَأَمَّا تَنْكِيرُهُ أَيْ الْمُسْنَدِ فَلِإِرَادَةِ عَدَمِ الْحَصْرِ وَالْعَهْدِ»
The predicate may be left indefinite to avoid restriction to one party or reference to an already identified instance.
«زَيْدٌ كَاتِبٌ»
Zayd is a writer.
The indefinite predicate «كاتب» attributes writing ability to Zayd. It does not by itself say that Zayd is the sole writer or refer to one previously identified writer known to the listener.
Do not mistake indefiniteness for an affirmative universal claim. The central point is the absence of a specific identification, not an assertion about every member of a category.