Relative restriction (قصر إضافي) depends on a particular contrast. “Zayd is a writer, not a poet” contrasts two attributes; “Zayd is in the house, not ʿAmr” contrasts two individuals. The exclusion must be read within the specified field of comparison.
The speaker may fully accept that Zayd has many other qualities; he is correcting a mistaken view only about poetry versus writing. Similarly, denying ʿAmr’s presence in one house makes no claim about other places or gatherings.
A reliable explanation should answer the question “as against which alternative?”. Without that information students may turn a limited exclusion into an unsupported absolute claim.