The book completes a point introduced in the preceding section: when words enter a construction, their specific rhetorical stations may differ. Even with a broadly related condition, in and idha are not interchangeable in every setting.
إِنْ قَامَ زَيْدٌ ...
If Zayd stands ...
إِذَا قَامَ زَيْدٌ ...
When Zayd stands ...
The former readily presents a condition as uncertain; the latter can emphasize an anticipated occurrence or temporal setting. These are rhetorical tendencies, and context is needed for a final interpretation.