I’m confused now.
I read that the restrictive relative clause provides essential information [strike]which function is[/strike] [which functions to/whose function is] to distinguish one from others.
Only restrictive relative clauses inside
definite noun phrases distinguish one referent from other
existing referents of the same category.
For example, we can’t write this way:
That man is Tom’s father who is drinking water.
If we do that way, that’ll indicate that Tom has another father who’s doing something else.
We should write this way:
That man is Tom’s father, who is drinking water.
Yes, and that is because Tom's father is a definite noun phrase. The following noun phrase is indefinite and does not require a comma:
That man is a father who is drinking water.
Another example:
Tom lent me a book which is in English.
This sentence indicates it was an English book that Tom lent me, not a Chinese book or Japanese book.
Yes, and notice that there is no need for a comma before "which is in English." "A book" does not have a unique referent, unlike "Tom's father," which does.
Consider the following three sentences:
(1) Tom went to a Japanese restaurant downtown which serves sake bombs.
(2) Tom went to the Japanese restaurant downtown, which serves sake bombs.
(3) Tom went to the Japanese restaurant downtown which serves sake bombs.
Sentence (1) carries no implication about the number of Japanese restaurants downtown; there could be one, and there could be more.
Sentences (2) and (3) do carry implications about the number of Japanese restaurants downtown; in (2), there is one, and in (3), more than one.
The crucial difference between (1), on the one hand, and (2) and (3), on the other, is that only "a Japanese restaurant downtown" is indefinite.
"A restaurant" will remain an indefinite noun phrase no matter how many adjuncts you add to it, be they adjectives, attributive nouns, or relative clauses.
"The restaurant" will remain a definite noun phrase no matter how many adjuncts you add to it.
At some point, the adjuncts of a definite noun phrase will yield a unique referent in the context. At that point, further adjuncts will be nonrestrictive.