The federation between mastodon and lemmy is strange. If a M account wants to follow a L community, they need to follow an automated M account which represents the L community. But if any M post mentions that L community, the post will get boosted by the community’s M account, so everybody who follows will get a notification. And I’m not sure if this can be moderated from the L side, because it seems like it never goes through L. Such as - do you see this @opensource ? Does a L mod see this?
The first paragraph of a post sent from Mastodon (or Akkoma, Misskey etc.) to a Lemmy community will appear in Lemmy as a headline. The character limit is 200 characters, only if it is longer will it be cut off. The post itself is displayed in full. However, only one image is federated, for example.