مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is only a writer.
Contrast the quoted sentence with مَا كَاتِبٌ إِلَّا زَيْدٌ (“No one is a writer except Zayd”). The first restricts qualities attributed to Zayd; the second restricts the persons to whom writing can be attributed.
The same negative-and-exception form may express either direction. Identify the predicated meaning and the subject being singled out; do not classify the direction solely by the presence of mā and illā.
StatementDirectionDoes NOT entail
Zayd is only a writerSubject → attributeNobody else writes
Only Zayd is a writerAttribute → subjectZayd has no other attributes
Only Zayd is in the houseAttribute → subjectZayd is never elsewhere