"Mutuals" apparently refers to the Relationship named "Mutual friends" where you both following each other.
"Followers" refers to the Relationship named "Followers" which is anyone who is following you, including "Mutuals."
"Friends" refers to the Circle named "Friends" which includes anyone you've put in "Friends" and is also the Circle in which "Followers" automatically go.
So who are "Subscribers" when you Approve a Contact?
This appears to be the ONLY place you ever see the word "Subscriber." There IS neither a Circle nor a Relationship called "Subscriber."
If you select that and hit "Approve" the person is added to your "Friends" Circle and placed under the "Follower" Relationship. So "Subscriber" = "Follower." Same thing.
"Groups" appears to refer to the Circle named "Groups" and NOT the Account Type named "Groups." However, when I tried to scope a post's "Permissions" to just "Groups" the Group I was following could not see the post. Not even if I @ mentioned them in it. So I guess "Groups" as a scope does not work.
I also set up a test "Organization" account. After following it I created a custom Circle named "Pages" and put that Organization in that Circle. When I posted something scoped to "Pages" the Organization account did see the post. So that appears to work, if for some reason you wanted to post something every Organization you were following should see. Though in the case of both an Organization or Group it's probably far more useful to just scope the post to their account name.
I also moved one "Follower" account from "Friends" into a custom Circle named "Acquaintances" and then made a post to all Followers but excluded Acquaintances and that worked as expected. That one Contact could not see the post but the rest of the "Followers" could see that post. So I gather from that your Circles take precedence over Relationships.
And that's one of the points of confusion with this. Some of the terms are referring to Relationships, some to Account Types, and some to Circles.
Notably, NONE of them appear equal to Facebook's "Friends of Friends." It seems the ONLY way to scope a post's reach to anyone you are NOT connected to is to make the post "Public."
If I'm wrong about any of this someone please let me know.
