Go to *Plugin* -> *Attribute Uniqueness*  and click *Add Config* button.

When you configure the Attribute Uniqueness {color:red}over object classes{color}, the configuration window requires you to set *Subtrees* field. This is incorrect, because only the *Top Entry OC* and *Subtree Entry's OC* fields are required.

The configuration window will not let you save  the configuration without the *Subtrees* field set. See the screenshot for more details. 

Corresponding command in the command line if you configure the plug-in over object classes is the fallowing:

dsconf -D "cn=Directory Manager" ldap://server.example.com plugin attr-uniq add "Mail Uniqueness with OC" --attr-name mail --subtree-entries-oc=inetOrgPerson --top-entry-oc=nsContainer

389-ds-base-2.5.2-2.el9_5.x86_64
cockpit-389-ds-2.5.1-2.module+el9dsrv+22071+e756ccfb.noarch
 

Chrome

1.
2.
3.

The configuration window lets you to save the plug-in config with only *Top Entry OC* and *Subtree Entry's OC* fields set.