The Databases Available field is populated with databases available to the search engine. You can select 0 or more database. If you select 0, all databases are queried.
To restrict the search to a specific set of databases, click a database name in the Available list, then click the right arrow to move that database to the Selected list.
To remove a database from a search, click the database name in the Selected list, then click the left arrow to remove that database name from the list.
