«قَصْرُ الصِّفَةِ عَلَى الْمَوْصُوفِ»
Restricting an attribute to one particular subject.
This reverses the direction of the argument. We no longer say that Zayd is a writer rather than a poet; instead, we say that among the relevant people only Zayd possesses a particular role or attribute. In “mā fī al-dāri illā Zayd” the property of being in the house is assigned solely to Zayd.
Nothing in that construction denies other attributes of Zayd. It limits who bears a particular predicate, whereas restricting a subject to an attribute limits which predicate is selected for that subject.