Why are some comments 'highlighted' while others aren't?

New-ish Lemmy user here (a couple weeks), I've noticed some comments under posts are 'lighter' than other comments. What causes this?

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I've wondered the same and thought they might be new since my last visit?
- I just entered the thread, so in that case all the comments should be highlighted.
So they're supposed to mark the new comments that we're created less the 10 mins ago. I'm not sure why you're seeing the other comment highlighted when it's a few hours old, did you follow a link to that specific comment? If so, that comment will also be highlighted
- I didn't follow any links.
  
  Update: the limit seems to be 4 hours for comments, not 10 minutes.

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