«وَالثَّانِي: الْفَرْدُ النِّسْبِيُّ»
The second category is relative singularity.
«لِكَوْنِ التَّفَرُّدِ فِيهِ حَصَلَ بِالنِّسْبَةِ إِلَى شَخْصٍ مُعَيَّنٍ»
Because the isolated transmission arises relative to a particular person.
A report may have several overall routes and be generally known, but one narrator alone may transmit it from a particular teacher. We must therefore ask “alone in relation to whom?”
Example: Three Successors transmit a report from one Companion, but only transmitter D takes it from one of those three Successors. D is singular relative to that teacher, even though other routes carry the report. This is an appropriate context for fard nisbi.