Ordinary qualification adds a time, place, condition, or other limitation to a statement; it does not necessarily exclude all alternative subjects. Qasr employs a recognized construction to convey an exclusive relation.
“Zayd is present today” specifies time without denying the presence of anyone else.
“Only Zayd is present” excludes competing people from the relevant assertion of presence.
The scope of exclusion is crucial. Some exclusions are real and unrestricted; others concern only alternatives being considered by the audience. This leads to real and relative qasr.