Site Collection Administrators

The following is a repost – reference – thank you Per Jakobsen
and more thx mike reference
The text is due to confusion SharePoint creates by having three different groups which when you create a site collection start out having the same persons, but are in fact different.
The three groups are:
Site Collection Owners

• Are controlled via `Central Administration | Application Management | Change site collection administrators'
• Receive email about site collection: quota warnings, not used warnings ...
• Automatically members of Site Collection Administrators
• Can only be users

Site Collection Administrators

• Are controlled via Site Settings | Site collection administrators
• Have full permissions in the site collection
• Can be users or groups

Site Owners

• A group named {SiteName} owners
• By default have full permissions in the site collection, but are subject to changes in permissions, broken inheritance, only read/edit own items,...
• Is just a standard SharePoint group which can be modified, deleted
• Can be users or groups

Strange things
Part of the problem understanding these groups are some of the strange behaviours of these groups:

• When a site collection is created you specify "Primary Site Collection Administrator" and "Secondary Site Collection Administrator" these two persons are entered into all three groups
• When you add "Site Collection Administrators" in Central Administration these can only be users and there can only be max 2 and they become both "Site Collection Owners" and "Site Collection Administrators"
• When you remove a "Site Collection Administrator" in Central Administration he/she is removed from "Site Collection Owners" and "Site Collection Administrators"
• When you add "Site Collection Administrators" in Site Settings in the Site Collection you can specify any number of users and/or groups they only become "Site Collection Administrators"
• If you remove a person who is "Site Collection Owner" as part of modifying "Site Collection Administrators" in Site Settings in the Site Collection he/she is removed from "Site Collection Owners" as well. If it's the Primary Owner then the Secondary Owner becomes Primary Owner, if there is no Secondary Owner, the first user in the list of Site Collection Administrators become new Primary Owner