In restriction of an attribute to a subject, one predicate is confined to a subject. In restriction of a subject to an attribute, the subject is confined, for the relevant comparison, to an attribute.
مَا ضَرَبَ عَمْرًا إِلَّا زَيْدٌ
No one except Zayd struck ʿAmr.
The specified action is confined to Zayd.
مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is nothing but a writer, in the relevant sense.
The second statement need not literally deny every other human quality; it often rules out the alternative roles or properties under consideration.