The source further clarifies the relation:
والنسبة بين الجزئي الحقيقي وبين هذا الجزئي المسمى بالجزئي الإضافي عموم وخصوص مطلقا؛
Between the real particular and the particular called relative there is absolute generality and specificity.
Zayd is a real particular, because he is a specified individual. He is also relatively particular, since the individual Zayd falls under the wider concept human.
Both descriptions therefore apply to Zayd, while “human” exemplifies relative particularity without being a real particular. This illustrates one-way inclusion between the classifications.