وَفِي الصِّفَةِ، أَعْنِي اسْمَ الْفَاعِلِ وَالْمَفْعُولِ وَغَيْرَهُمَا مُطْلَقًا.
Translation: Also in descriptive words, namely active and passive participles and other such forms.
The source also allows concealed subject pronouns in active participles, passive participles and related descriptive forms.
زَيْدٌ قَائِمٌ.
Zayd is the nominal subject; qāʾimun is the nominative predicate; the participle’s implicit doer is a concealed huwa referring to Zayd.