The source gives the proper name Zayd as an example of a particular, assuming a specific Zayd is intended.
'Human' can apply to Zayd, Amr and Bakr.
'Zayd', referring to that one specified person, cannot be replaced by Amr or Bakr as the same individual.
Many people might happen to bear the name Zayd; that does not make a particular use of the name refer to all of them.
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The textbook’s example: Zayd
Universal and Particular — Definitions and Practical Classification
The source gives the proper name Zayd as an example of a particular, assuming a specific Zayd is intended.
'Human' can apply to Zayd, Amr and Bakr.
'Zayd', referring to that one specified person, cannot be replaced by Amr or Bakr as the same individual.
Many people might happen to bear the name Zayd; that does not make a particular use of the name refer to all of them.