مَا فِي الدَّارِ إِلَّا زَيْدٌ
Only Zayd is in the house.
This confines the state of being inside the house to Zayd. It does not confine Zayd himself to that one state. He can also be learned, generous, or possess other attributes.
Attribute-to-subject qasr excludes other bearers of the specified quality. It does not imply that the named subject has no other qualities. The distinction prevents the two directions of qasr from being mistakenly exchanged.
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Qasr: Definitions, Real and Relative Restriction, and Ifrād, Qalb and Taʿyīn
مَا فِي الدَّارِ إِلَّا زَيْدٌ
Only Zayd is in the house.
This confines the state of being inside the house to Zayd. It does not confine Zayd himself to that one state. He can also be learned, generous, or possess other attributes.
Attribute-to-subject qasr excludes other bearers of the specified quality. It does not imply that the named subject has no other qualities. The distinction prevents the two directions of qasr from being mistakenly exchanged.