الْمَعْنَى الْقَائِمُ بِالْغَيْرِ
A meaning that inheres in or is predicated of another.
The word ṣifah in this rhetorical discussion is not confined to an adjective (naʿt) in syntax. Being a writer, standing, or being in a house all qualify as meanings attributed to a subject.
The mawṣūf is the thing about which that meaning is asserted. Thus “in the house” is not a grammatical adjective in “No one is in the house except Zayd,” yet the relevant state functions as the attribute in rhetorical qasr.