sentential modifiers(?) and relative clause question


I just finished Japanese level 2 in college, which as I have come to
understand is ~the most important~ and I came away with quite a lot of
questions.

We learned about relative clauses and something my teacher referred to as a
"sentential modifier" and they seem very similar, but clearly have different
functions. Hoping someone could help.

> コーヒーを飲んでいる男の人です。
>
> the man drinking coffee

So as I understand this is an example of a relative clause (i.e. the preceding
clause modifies the noun). However, when my sensei brought up a "sentential
modifier" she kind of glossed over it. But here was the sentence

> これは私が使う水です。
>
> That is the water I use.

So, these things are pretty similar, but I was hoping someone could clarify
this for me. Both sentences use the short form of verbs and no particle
between the verb and the ~leading~ noun.

If these are both correct, how commonly are clauses/sentences like this used?
Are there any "rules" about use of particles in these instances?




  * S1. コーヒーを飲んでいる男の人です。 == Here's a man (who is) drinking coffee. ( a relative clause ? )

  * S2. これは私が使う水です。 == This is the water [that I use].

The latter is said to contain a "sentential modifier" because instead of [that
I use], it could be [that Peter uses] or (based on) any elaborate sentence.

e.g. 「これは、頭が赤い魚を食べた猫と私が飲む水です。」

I think the main difference is whether the [relative clause] contains a
subject or not -- (I mean "subject" in the sense of English grammar.)

