One use of the article refers back to something already mentioned. For instance:
«جَاءَنِي رَجُلٌ، فَأَكْرَمْتُ الرَّجُلَ» — A man came to me, and I honored the man.
The second occurrence of «الرَّجُلَ» is linked to the individual introduced by «رَجُلٌ». The article's role is therefore not merely a formal marker of definiteness; it unifies the reference across the exchange.
The source contrasts prior-reference definiteness with proper naming. The former depends on information supplied or assumed in context; the latter is grounded in an assigned name. The sentence above is an explanatory illustration, not an exact quotation claimed to be in the textbook.