similar discussions...similar to what?

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Hi, I am quite new to this site, and something is still unclear to me. How does the "similar discussions" section work? I have often looked at the other discussions it shows us, when reading a discussion, and have never found any similarity. The tag words aren't the same, the titles aren't the same, the authors aren't the same, the topic isn't the same. Can someone explain to me how the site choses the discussions that are similar to the one we're reading? Olivier