«قَصْرُ الْمَوْصُوفِ عَلَى الصِّفَةِ»
Restricting the described subject to an attribute.
Here the subject (الموصوف) is restricted to a particular quality instead of competing qualities. If “mā Zaydun illā kātib” is interpreted as absolutely real restriction, it would deny every attribute of Zayd except writing—an untenable claim about an ordinary human being.
If the listener is deciding specifically between poetry and writing, the same wording can have relative restriction: Zayd is a writer, not a poet, within that contrast. The expression may remain the same while its rhetorical classification changes with the intended range.