from the standpoint of an additional distinction—such as being early to embrace Islam or witnessing the eminent campaigns (1)—divides them into separate classes.
This is the method favored by the author of al-Ṭabaqāt, Abū ʿAbd Allāh Muḥammad ibn Saʿd al-Baghdādī (2), and his book is the most comprehensive compilation on the subject.
The same difference of approach applies to those who came after the Companions, namely the Successors: whoever (3) considers them solely (4) in terms of having received reports from some of the Companions regards them all as a single class, just as Ibn Ḥibbān did; whereas whoever evaluates them on the basis of direct meeting (5) divides them, as (6) Muḥammad ibn Saʿd (7) did. Each of these two approaches (8) has its own justification.
[Knowledge of the Birth and Death Dates of Transmitters]
Among the important matters as well is to know their births (9) and deaths; for by knowing these dates (10) one is safeguarded from any claim (11) that certain transmitters met (12) when in reality they could not have done so.