قَصْرُ تَعْيِينٍ إِذَا اعْتَقَدَ وَاحِدًا غَيْرَ مُعَيَّنٍ
Ta‘yin applies when the listener knows one of two alternatives holds but cannot identify which.
For instance, a person knows that Amr is either a doctor or an engineer but cannot tell which. The answer مَا عَمْرٌو إِلَّا طَبِيبٌ establishes that Amr is a doctor, not an engineer, in that contrast.
The listener neither combined both alternatives nor firmly held the opposite judgment. The problem was indeterminate choice, and the restriction settles it.