مَا زَيْدٌ إِلَّا كَاتِبٌ
Zayd is the writer among the options.
If the listener does not know whether Zayd is a poet or a writer and seeks a decision, “Zayd is only a writer” resolves that uncertainty. This is qasr al-taʿyīn.
In qalb a definite but wrong alternative is believed; in ifrād two alternatives are wrongly combined; in taʿyīn the listener is undecided between them. The construction’s rhetorical type follows the prior epistemic state.