وبالعلمية أي تعريف المسند إليه بإيراده علماً
Defining the subject by proper-noun status means bringing the subject into the utterance as a proper name.
A proper name (العَلَم) is assigned to a particular individual or referent. Rhetorically it brings that referent forward by name, rather than merely describing a general property.
In «زَيْدٌ مُجْتَهِدٌ», Zayd is the proper name and ‘diligent’ is the predicate. In «رَجُلٌ مُجْتَهِدٌ», ‘a man’ does not function as that individual's proper name. The two utterances therefore identify their subjects differently. What rhetorical advantage is gained depends on the surrounding context.