القصر
Restriction or confinement of a judgment to a particular range.
Qasr adds a concern beyond simply reporting something: it marks an exclusive relationship or excludes a competing possibility. Hence it receives a separate chapter.
For example:
مَا جَاءَ إِلَّا زَيْدٌ — None came except Zayd.
This reports more than Zayd's arrival; it confines the arrival to him. The types and conditions of such restriction belong to the later chapter on qasr.