Definition of Unique Fields
When you have installed Virto SharePoint Custom Mask and Unique Field Validator, you will see the feature in the list settings of your current list. Go to “Settings – List Settings” and click “Unique field settings”.
In case you need to define a field as a unique one, check the box opposite the required field and select the scope (the level which it will be unique on - current list, site or site collection). For example, make “Title” field unique. Then click “OK”.
Now if anyone enters a value that already exists in the current list (or other scope), the system will not allow to save this entry – you will see “Duplicate values” message.
This way you can define settings of unique fields and apply them to your SharePoint lists.