>>285186570
Basically, public and friends+ worlds are considered "Public" and you aren't meant to be using anything NSFW.
Friends-only, Invite+ and Invite Only are considered private. Friends-only does exactly what it says on the box, Invite+ means anyone in the world can invite people, Invite only means only the host can invite, but other people can request the host invite people.
Invites don't always work reliably though, so have fun. Also, there's no way to index f+/f/i+/i, you need to know people or use a portal.