I have read extensively concerning the rules governing that vs. which and felt I understood them until the following example. I am operating under the assumption that "that" is used for restrictive clauses and "which" is used for non-restrictive clauses. Despite this I am torn on the following example:

“Astronaut suits contain dozens of materials THAT separate an astronaut from space.”

It seems to me the phrase "separate an astronaut from space" is important to the meaning of the sentence, and therefore "that" should be used. Is this correct? How does one come to the correct conclusion in a short sentence like this?

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    With integrated (defining) relatives, it's generally a free choice between the wh- kind and that-relatives. "Materials that separate an astronaut from space" and "materials which separate an astronaut from space" show no semantic differences, and no syntactic differences other than what follows from "that" not being a pronoun but a subordinator.
    – BillJ
    Feb 17 at 16:46
  • Why do you feel sentence length matters? Your sentence is not short. Feb 17 at 16:49
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  • I am used to examples such as "diamonds, which are expensive, are my favorite" for which it is shown that "diamonds are my favorite" is the main idea. I suppose rather than the sentence being short, I am trying to communicate that the sentence has no information after the "that" phrase. Most examples I see advise considering taking the phrase in question out and seeing if the meaning stays the same -- I am unsure how to apply that in this case. Feb 17 at 16:50
  • @yourmoveantonius Now you're talking about supplementary (non-defining) relatives, the kind introduced by a wh pronoun but not (normally) by "that". The supplementary kind adds extra non-integrated information that is not needed to delimit the set denoted by the antecedent. Having said that, surely your example can only be of the integrated kind?
    – BillJ
    Feb 17 at 17:14

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