وَيَخْتَصُّ بِالْمَقْصُورِ، وَهُوَ مَا فِي آخِرِهِ أَلِفٌ مَقْصُورَةٌ، كَـ«عَصَا».
Translation: This applies to the maqṣūr noun, a noun ending in a final alif, such as عَصَا.
Explanation: The three case vowels cannot be pronounced on that final alif, so they are estimated.