Mutuals refers to the Relationship "Mutual friends" where you are both following one another.
Followers refers to the Relationship "Followers" which is anyone who if following you, including "Mutuals."
Friends refers to the Circle named "Friends" which includes anyone you put in that Circle. When you add a Contact this is the default Circle they are put in, so it also includes your "Followers" and "Mutuals."
I also moved one Follower from "Friends" to a custom Circle named "Acquaintances" and made a post to "Followers" but excluded "Acquaintances" and that worked as intended. That one Follower could not see the post but the rest could. So it appears that Circles take precedence over Relationships.
Groups refers to the Circle named "Groups" not the Account Type called "Groups." But as far as I could tell no group in your Contacts can see anything posted with only this scope, even if you @ mention them specifically. You're better off scoping the post to the specific group account.
I also created an "Organization" sub-account. In another test account I created a custom "Pages" Circle. Then I made a post scoped to "Pages" and, yes, the Organization account could see that post.
So what, then, is a "Subscriber"?
The connection request is, so far as I can tell, the ONLY place you ever see "Subscriber." It is not a Circle, Relationship, nor Account Type. If you select "Subscriber" and hit "Approve" the person is added to your Friends Circle and your Followers Relationship. So "Subscriber" = "Follower." Same thing.
There does not appear to be anything similar to Facebook's "Friends of Friends." It looks like the ONLY way to scope a post to accounts that are not in your Contacts is to make the post "Public."
