English commonly uses "that-"clauses for defining purposes and "which"-clauses, enclosed in commas, for non-defining descriptive purposes.Here is an example where using "that" or "which" give quite different meanings:

"He criticised the bad behaviour of the children, which was most conspicuous" (All their bad behaviour was conspicuous);
"He criticised the bad behaviour of the children that was most conspicuous" (He confined himself to criticising only the bad behaviour that was really obvious).

Here is one sentence with a non-defining followed by a pair of defining clauses: "He played the piece, which was quite difficult, in the exact manner that I play it myself, but on an instrument that I wouldn't even accept as a gift."

Here are two other examples where the commas change the meaning:

"My financial adviser, who lives in New York, has just written me a letter" (non-defining clause);
"My financial adviser who lives in New York has just written me a letter" (defining clause - i e I have other financial advisers elsewhere).

"He has two sisters, who work in the Ministry" (non-defining clause; he has only two sisters);
"He has two sisters who work in the Ministry" (defining clause; presumably he has other sisters who work elsewhere).

As usual in English, you will find exceptions to this rule of thumb. You will also find English people who misuse "that" and "which".
All the same, checking whether the clause is defining or non-defining can be a useful guide. Can you remove the clause without making total nonsense of the sentence? If you can, it is a non-defining clause.