Comment: Question for mods

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Question for mods

I asked earlier in the thread but have not yet seen an answer so am renewing the question so it doesn't get lost in the flow before a mod has time to respond.
I tried reading the FAQs but did not find an answer to my question...

What are the rules, guidelines, concerning comments being collapsed?

I am aware that I have an option to collapse ALL comments under a thread but why are some comments pre-collapsed within the threads? At first I thought it was automatic after a comment received a quota of "down" votes but that is not always the case.

No emergency, but would appreciate an explanation (or directions to where the needed info can be located) as to how the site works concerning the comment collapse feature. Thanks.