Usually in books of jarh wa ta'adil scholars classify narrators in so called tabaqat الطبقات which seem to be some kind of categories or division for narrators. I found it strange that two people who at least have been contemporary of a certain time may have a big gap in their so called tabaqah for example one scholar was assigned to the 7th tabaqah while the other was in the 10th.
Therefore I'd like to know how is this term defined and how to interpret it?
