Whole-object substitution: «حضر أخوك زيدٌ» names the brother.
Partitive substitution: «قرأتُ الكتابَ نصفَهُ» specifies the half read.
Inclusion substitution: «أعجبني زيدٌ علمُهُ» specifies Zayd’s knowledge as what is admired.
These three illustrative grammatical types show how substitution can specify the whole referent, one of its parts, or an associated property. In each, the added expression changes what exactly the hearer takes as central.