“Which” and “that” are very commonly used in relative clauses.
In the following examples the underlined sections are the relative clauses:
Unfortunately we cannot open the documents that you emailed earlier as they have been corrupted.
The conduct of the Tender Committee, which has not yet explained the motives for its decision, infringes the Bidder’s legal interest.
In relative clauses, “that” and “which” are called relative pronouns.
The first example above contains a defining relative clause. This means that the relative clause defines the thing mentioned before it (in this case, the documents), and the sentence would not make sense if the relative clause were omitted (we would not know which documents were being referred to).
Defining relative clauses have no commas around them and can use “that” as their relative pronoun.
The second example above contains a non-defining relative clause. This means that the relative clause does not define the thing mentioned before it (in this case, the Tender Committee), but instead gives additional information about it. Thus the relative clause can be omitted and the sentence will still make sense.
Non-defining relative clauses always need commas around them and CANNOT use “that” as their relative pronoun.
Defining relative clause:
This remuneration may not be less than the national minimum wage reduced in proportion to the hours of work which / that are to be performed in a given calendar month.
Non-defining relative clause:
The worst-case scenario, which we should make every effort to avoid, is that the court will register Acme, but will not register the Bank.