Starting with the Spring '18 release, Salesforce gave administrators control over the creation of List type Custom Settings. The "Manage List Custom Settings Type" option in Schema Settings is disabled by default, which causes the "List" option to appear greyed out in the Setting Type field when creating a new Custom Setting. Enabling this setting allows the creation of application-level data definitions using List Custom Setting types.
To allow users to create List Custom Setting types, enable the Manage List Custom Settings Type option in Schema Settings.
To verify, go to Setup > Home > Custom Code > Custom Settings, click New, and confirm the List option under Setting Type is no longer greyed out.
To verify, go to Setup > Develop > Custom Settings, click New, and confirm the List option under Setting Type is selectable.
Note: By enabling this option, all users with access to Custom Settings will be able to create List Custom Setting types. For more information, see the Schema Settings Help documentation.
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