What is the meaning of the statement of the scholars of Rijal (narrator criticism): "from the fourth" or "from the ninth" or "from the fifth"?

The term "tabaqah" (class/generation) refers to a generation of narrators who lived in the same era or shared the same teachers. Knowing the tabaqah of a narrator is useful for distinguishing between those with similar names or lineages, and for ascertaining whether a chain of narration is connected or断. What is meant by statements found in narrators' biographies, such as "...of the fourth (tabaqah)," or "...of the ninth," or "of the fifth...," is the narrator's tabaqah, according to the methodology and arrangement followed by Al-Hafiz Ibn Hajar in his book "Taqrib al-Tahdhib," in which he divided narrators into twelve tabaqat, starting with the Companions and ending with the younger students of the Tabi' al-Tabi'in (followers of the followers).