In riwāyat al-aqrān, a transmitter reports from a contemporary or someone of a similar scholarly generation. Peer status may involve age, period of hearing or scholarly association; death dates alone are not enough.
Mudabbaj requires that both peers narrate from each other: A from B and B from A. If only A narrates from B, peer transmission may be established, but mutual transmission has not been shown. Each direction needs its own evidence.
Example: Two contemporary teachers study in one city. One chain records Zayd reporting from ʿAmr; another independent chain records ʿAmr reporting from Zayd. This supports the mutual form. It does not by itself prove which scholar was stronger or weaker; the term describes a transmission relationship.