Have seem some topics have this notification at the bottom of replys, what is this used for?
And, is that a function can be enabled on self installed instances?
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Hey,
Its used mostly in topics where you have docs and you want to keep the topic free from old replies / clutter etc. After a set period of time you can automatically delete the replies. If a post has 5 or more likes it does not get deleted. This value can be changed by this setting skip auto delete reply likes
This feature can be used on self installed instance
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At the more frivolous end, it can be used for game topics too. We have some A-Z, Last Letter, etc chain-style games, and having a topic timer allows those to be cleared out automatically so they don’t bloat out. 
We’ve even used it as part of the game structure for a couple, where the timer is set for 1 day and the idea is to keep the ‘chain streak’ going for as long as possible before being wiped out back to the OP/stub post.
Probably not useful to more serious forums, but thought I’d throw it in for good measure. 
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Can this be enabled in a way that selected replies are kept?
The use case would be to build up a FAQ-like list of useful comments bellow while removing clutter, keeping the question and the answer or something like that.
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The ones with 5+ Likes are kept, as @ondrej explains above. However, you can copy and paste any useful information from the comments/replies and add it to the OP if that’s a better place for it.
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I can only find the setting for skip auto delete reply likes
Where can I turn on and change the # of days for auto deleting replies on a topic? Or, better yet, can it be set per category?
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Nevermind. I found it in the Topic Timer settings on each topic.
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