As mentioned in [Pragmatics - Section 1.3.] indefinites are used when the speaker assumes that the addressee is not familiar with a certain individual/thing. Indefinites are further divided into specific and non-specific. A speaker uses a specific indefinite when s/he knows the referent or has a certain individual/thing in mind and a non-specific indefinite when s/he does not know the referent or does not have a certain individual/thing in mind.
The indefinite forms described in [Pragmatics - Section 1.3.] can be used to express both specific and non-specific individuals/things. The difference is expressed through the presence vs. absence of non-manual marking of non-specificity which is described in [Pragmatics - Section 1.4.2.] below.