Community-only
The post stays with its members.
Universal feed
A community can make selected posts visible beyond its channels, without making its whole space public.
Visibility
The post stays with its members.
Signed-in people can discover it.
It gets a shareable page that needs no account.
Its author or community can ask for public indexing.
Discovery
Someone can read a post, discover its author, visit the space behind it and then join the conversation.