Oliver,
In the States, this is the difference in usage, and a quick explanation...

Restrictive clauses do not take a comma, and they use the word "that."
Example:

Dogs that have three legs need special medical care.

You can remember it is "restrictive" because the “that clause” restricts the
sentence to dogs that have three legs (i.e., it does NOT apply to all dogs).

Nonrestrictive clauses take a comma and use the word "which." Example:

Dogs, which are members of the canine family, are related to wolves and
foxes.

You can remember it is "nonrestrictive" because the “which clause” applies
to ALL dogs.

Making this distinction might seem meaningless, but in fact it is not, as
you can see in these two examples.

Senator Smith opposes new state taxes that will increase the cost of doing
business in Wisconsin. (He opposes only those taxes that increase the cost
of doing business.)

Senator Smith opposes new state taxes, which will increase the cost of doing
business in Wisconsin. (He opposes all of them.)

Hope this helps,
Corynne

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