Generally, the people who've made feeds and want other people to know how to post in them will mention that in the feed's description, but there may be other things that the feed is looking for that brings stuff in too. #UKMathsChat is a great example of this: all you have to do is add the tag, but…

as Catriona says, there are some other things that will end up in there too, depending on what the feed creator has decided might flush out relevant stuff that doesn't have the tag.