I am not sure whether "represent" is the right word here. What I mean is that all posts have a "recipient" (the audience).
For Mastodon, you have public posts where the recipient is literally a "special" audience, called https://www.w3.org/ns/activitystreams#Public. If you want to see a private message to alice, you just change the "audience" to contain only the https://example.com/alice actor URI.
To post to a community, it's the same logic: if you are posting on fediverse@lemmy.world, then the message has "https://lemmy.world/c/fediverse" as the audience. This message is then sent to lemmy.world and processed.