Once an entity has been established in the universe of discourse, it is treated as referring, regardless of what its status may be in the Real World. It concerns about a psychological dimension from speaker’s perspective, which is how strongly does the speaker intend to suggest that they are referring to a particular individual. Here are some examples with NPs marked by indefinite articles, i.e., ‘a’, ‘any’, ‘no’ and ‘some’.
1) An indefinite NP with ‘a’ marked can be interpreted as referring or non-referring:
e.g., I know she met a man at the bar. (referring)
She may meet a man there. (non-referring)
2) ‘Any’ marks only non-referring nouns:
e.g., She can see any man. / *She saw any man.
3) ‘No’ as the negation marker marks only non-referring nouns:
e.g., No one came to see her.
4) ‘Some’ may encode both referring and non-referring nouns:
e.g., I know she met some man there, and that she told him that….
I know she met some man there, and whoever it turns out to be will know what to do.
Givón, Talmy. 1993. English Grammar: A function-based introduction (Vol. 1 & 2). John Benjamins Publishing.