where, when ?

1. of such information 是否可放在 is useful of such information to users ??.
of such information is an adjective phrase to qualify users. of such information tells the readers what kind of users they are.

You cannot write it as is useful of such information to users.


2. Let's focus on the part represent faithfully that which it is said to represent.

Represent is a transitive verb, that is, represent must be followed by an object that can either be a noun phrase or a noun clause.

The object of the first represent is the noun clause that which it is said to represent.

To be more specific, that is the object of represent and which is a relative pronoun for the word that. This which is the object of the second represent in the sentence.

That part of the sentence can be rewritten as represent faithfully what it is supposed to represent.

In simpler terms, the data must represent what people believe it should represent.


3. where 可以用when 代替嗎?
No, where must be used. Where does not refer to a location but a condition.

1. 我認為原本咁樣已經ok，改成...is useful of such information to users，感覺怪怪的！

2. ...which it is said to..的意思是："即是說"。

3. 都ok的。