«إِنْ وُجِدَ بَعْدَ ظَنِّ كَوْنِهِ فَرْدًا قَدْ وَافَقَهُ غَيْرُهُ»
If, after a narration was thought to be solitary, another transmitter is found agreeing with it, a mutaba‘ah has been discovered.
Relative singularity (fard nisbi) means being alone only in a particular respect: from a specified teacher, in a generation, or under a certain qualification. A pupil may seem to be the only narrator from their own teacher even though another route through that teacher or a higher link exists.
A mutabi‘ is the parallel transmitter whose route supports the initial narration. The point where the chains meet determines which type of corroboration is present.