«وَيُسَمَّى قَصْرَ تَعْيِينٍ»
And it is called restriction by determination (taʿyīn).
Taʿyīn (قصر تعيين) identifies one option when the listener is undecided about the subject or attribute in question. The listener is not firmly committed to a wrong choice and does not necessarily believe that both alternatives hold together; the issue is uncertainty.
Asked “Did Zayd or ʿAmr write the article?”, the answer “It was Zayd alone” identifies the person. Distinguishing ifrād, qalb, and taʿyīn therefore often requires reconstructing the addressee’s previous understanding.