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.

$wgRestrictionLevels 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:

Note: 'sysop' and 'autoconfirmed' are actually user groups rather than user rights, but are allowed for backwards compatibility. They should be mapped to 'editprotected' and 'editsemiprotected' respectively when comparing against user rights.

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.