«وَإِنْ رَوَى الرَّاوِي عَمَّنْ هُوَ دُونَهُ فِي السِّنِّ، أَوْ فِي اللُّقِيِّ، أَوْ فِي الْمِقْدَارِ»
“When a transmitter narrates from someone beneath him in age, encounter, or standing.”
This is riwāyat al-akābir ʿan al-aṣāghir: a senior person reporting from one junior to him in some relevant respect. The chain runs contrary to the common assumption that transmission always moves from the older or higher-ranked teacher to the younger or lower-ranked pupil.
The conjunction “or” matters. The junior need not be younger in all three respects. A difference in age, the generation of elders encountered, or scholarly standing can be sufficient, depending on the case.