«رجل جاءني»
A man came to me.
An indefinite noun in subject position can make its kind or category salient—perhaps the point is that the entrant was a man rather than a woman. Yet the sentence does not in every context logically rule out the arrival of women.
The source cautions against automatically reading exclusive classification into an initial indefinite. Such an assumption would add unexpressed restrictions to many acceptable Arabic sentences.
The correct analysis asks whether the discourse actually contrasts categories, or whether the initial noun merely introduces the participant.