A secondary field appears below the primary record name in instant results for lookups.
The field provides more contextual information to help users select the right record. For example,
the location San Francisco appears under the company Acme to help users distinguish
similar record names. Or a phone number appears under account and contact records. This feature is new in all versions of the Salesforce1 mobile
You can determine which secondary field appears to users by customizing the search layout. The
second usable field in the Search Results layout appears as the secondary field in instant
results. Examples of unusable fields are formula fields, HTML-formatted fields, inline image
fields, picklists, or long-text fields.