وَيَخْتَصُّ بِالِاسْمِ الْمُفْرَدِ الْمُنْصَرِفِ الصَّحِيحِ.
Translation: This rule particularly applies to the singular, fully declinable, sound-final noun.
The author then brings two further groups under the same pattern: nouns treated like sound-final nouns and fully declinable broken plurals.
“Particularly” identifies the groups in the book's first class; it does not mean that these case signs can never occur elsewhere.