After enabling enhanced lookups, specify which fields users can use to filter lookup search results. If you don't specify any fields, your users can't use filters in enhanced lookup dialogs.
Available in: Salesforce Classic
Available in: All Editions except Database.com
User Permissions Needed
To specify lookup filter fields:
From the management settings for accounts, contacts, opportunities, users, or custom objects, go to Search Layouts.
For the Lookup Filter Fields layout, click Edit.
Use the arrows to add or remove fields from the layout and to define the order in which the fields should display. You can add up to six filter fields to the Selected Fields list. To select more than one field, use CTRL+click, or SHIFT+click to select multiple items in a range.