Isnād (إسناد) establishes a predicative relation between two sides. In “نَجَحَ الطَّالِبُ” success is assigned to the student; in “كَتَبَ زَيْدٌ الرِّسَالَةَ” the writing is assigned to Zayd, while the letter is the object.
«ضَرَبَ زَيْدٌ عَمْرًا»
Zayd struck ʿAmr.
The action is attributed to the grammatical subject, Zayd, not the object, ʿAmr.
Also distinguish a possessive phrase, «كتاب زيد» (Zayd's book), from a verbal predication, «كتب زيد» (Zayd wrote). The present definition is concerned specifically with assigning an action or its meaning.