definiteness in Rioplatense Spanish

Rioplatense Spanish marks definiteness on a determiner rather than on the noun: el, la, los and las present a referent as identifiable, and un, una, unos and unas present it as not identifiable. A countable noun in the singular cannot stand as an argument without one of them, which is why the value is recorded as obligatory, while bare plurals and bare mass nouns are the contexts in which the distinction does not apply. The finer contrast between a particular referent the speaker has in mind and no particular referent at all is not carried by the determiner, which is the same in both readings; it surfaces instead in the mood of a following relative clause, so busco un libro que tiene ese mapa presents a particular book and busco un libro que tenga ese mapa presents none. The definite determiner also has a use with no noun at all, as in lo importante, which is a separate matter from the definiteness of a noun phrase.