قَصْرُ إِفْرَادٍ إِذَا اعْتَقَدَ الْمُخَاطَبُ الشِّرْكَةَ
Qasr of ifrad addresses a listener who assumes two competing things coexist.
Suppose the listener believes Zayd to be both a poet and an orator. One may answer: مَا زَيْدٌ إِلَّا خَطِيبٌ — in this contrast, Zayd is an orator, not also a poet.
The reply affirms oratory and negates the assumed addition of poetry. Ifrad corrects a supposed combination of two alternatives, rather than simply supplying information to a person who has no prior belief.