وَيَدْخُلُ بَيْنَ الْمُبْتَدَأِ وَالْخَبَرِ صِيغَةُ مَرْفُوعٍ مُنْفَصِلٍ مُطَابِقٍ لِلْمُبْتَدَأِ، إِذَا كَانَ الْخَبَرُ مَعْرِفَةً أَوْ أَفْعَلَ مِنْ كَذَا، وَيُسَمَّى فَصْلًا؛ لِأَنَّهُ يَفْصِلُ بَيْنَ الْخَبَرِ وَالصِّفَةِ.
Translation: A matching independent nominative pronoun may intervene between the initial subject and its predicate, when the predicate is definite or comparative, to distinguish the predicate from an adjective.
The source uses the independent nominative form between initial subject and predicate, matching the subject. Where the predicate is definite or comparative, the form clarifies that the following expression is the predicate rather than a modifier.