Manual:$wgRestrictionLevels

Details
This variable contains a list of permission keys that can be selected for each restriction type on the "page protection" page (action=protect). 'autoconfirmed' allows all registered users if $wgAutoConfirmAge and $wgAutoConfirmCount are 0.

Note that this variable holds a list of user permissions/rights, not user groups. If you need to, you can make up your own permission and then assign it to the different user groups as required. For example:

If you add additional levels (e.g. 'docwriters'), you can set the "display name" seen in the user interface by creating a MediaWiki page like 'MediaWiki:Protect-level-docwriters'.

If you set a level higher than sysops, that is, protection from sysop editing, sysops cannot give a page that level of protection nor remove it, even with the 'protect' permission.

A user with the 'protect' permission still cannot protect a page that he/she can't edit, such as if defined as such by $wgNamespaceProtection.