@McTag,
Quote:It is a direct quotation from the Oxford usage book mentioned above.
Oxford: "The general rule is that 'which' is used in relative clauses to which the readers' attention is to be drawn, while 'that' is used in clauses which mention what is already known or does not need special emphasis."
I truly hope that you have quoted it out of context. As it stands it's pretty much a mindless quotation.
Both THAT and WHICH are used in restrictive relative clauses.
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M-W: restrictive clause
a descriptive clause that is essential to the definiteness of the word it modifies (as that you ordered in “the book that you ordered is out of print”)
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WHICH is used in non-restrictive relative clauses.
Non-restrictive relative clauses are used to add extra information about a noun referent but that information is not essential in determining which item is being described.
Quote:No, and that is why the additional clause, which adds information, is a non-restrictive clause as defined (in the same publication).
All relative clauses add information. Some is essential to meaning and some is superfluous addition.
The pen
that/which I'm holding is blue. This pen,
which has a name embossed on it, was swiped from McTag's desk.
In the first sentence of the above, THAT or WHICH are used as relative pronouns heading the restrictive relative clause "I'm holding". That clause is essential to defining or limiting the discussion to a certain pen. It restricts our focus to the pen "I'm holding". That's why it is a restrictive clause.
In the second sentence of the above, WHICH heads the non-restrictive relative clause, "has a name embossed on it", which is extra, unnecessary information which is not essential to determining the pen in question. That non-restrictive phrase could be dropped without affecting anyone's knowledge as to which pen is under discussion.
We know it is non-restrictive because non-restrictive clauses are put in commas and their head relative pronoun is almost always WHICH.
That is the "general" rule followed by all native speakers of English.
We will ensure that you are presented with quality experiences which challenge you, without overwhelming you."
"experiences" is limited by the restrictive relative clause, which is headed in this particular case by WHICH, to those that "challenge you". Without that restrictive clause we wouldn't know the limit on the 'experiences'.