@finitebanjo@feddit.online explained pretty good but I want to reiterate that the instance ultimately dictates what is allowed or not. the software has defaults on some blocks and defederations but its up to the instance maintainers to keep the defaults or change that. each community then may or may not have rules. communities do not by default federate community posts the way most of the software works. It needs a user of the instance to subscribe to the community. piefed did something to allow auto subscribing to sorta fix that and its possible that has made its way out since most all the ecosystem is open source but im not sure. there are often multiple communities that have the same subject and often on different instances. some communities seem to limit or do not federate at all. Like the dedicated nsfw instance and an anime instance has communities that people only see if they have an account on the anime instance. this I think came about from changes that allowed for private communities. a bit annoying for me though the lazy bum who does not want to make multiple accounts but does want to see it all. my own instance has all the piefed defaults. its kinda stage for piefed but pretend I did not say that as it kinda rubs people the wrong way. but it generally gets the new stuff first. I don't change because again lazy and honestly the piefed defaults when I look them up pretty much line up with things im fine not seeing. for pure philosophical reasons I should move but man I like getting the new stuff fast and I like how piefed does stuff.