I'm always confused about using "that" and "which" in a sentence. I have learned that "which" is used in non-defining clauses (this clause can be omitted), while "that" is used in defining clauses and cannot omitted. However, I cannot decide which one to select in my sentence:

This structure consists of 2 layers: a physical layer ...that/which... contains sensors and a logical layer ...that/which... contains logic.

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    'Which' sounds considerably better to me as a native speaker in both cases. I think it is because the clause is just adding information to describe the physical layer, and so you could leave it out or put it in brackets. – Jacob Lee-Hart Jan 23 at 17:22
  • It's not entirely clear whether the relative clauses in your example are intended to be defining or non-defining. Please clarify. – BillJ Jan 23 at 17:45
  • @BillJ I'm not sure. In the first part I'm talking about the components of a physical layer, so it seems to be defining, am I right? – Pablo Jan 23 at 17:57
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    OK, so they are non-defining relatives, in which case only "which" is possible. You can't use "that" in non-defining relatives. – BillJ Jan 23 at 18:09
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    It is probably worth noting that a clearer way to structure this sentence would actually eliminate the relatives altogether: This structure consists of two layers: a physical layer containing sensors and a logical layer containing logic. – Orbital Aussie Jan 24 at 5:00

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