The book combines three different descriptions of the same noun:
Singular: referring to one person or thing, such as زَيْدٌ.
Fully declinable (munṣarif): normally taking tanwīn and kasra in the genitive, as زَيْدٌ، زَيْدًا، زَيْدٍ do.
Sound-final: not ending in a weak letter, as in زَيْدٌ.
Important: being singular alone is not enough. The book also specifies fully declinable and sound-final.