Note: this article discusses search in the context of Salesforce Classic; search within the Lightning Experience is discussed in the Chatter section.
Global Search is automatically enabled when Chatter is enabled. It cannot be enabled otherwise. As you can see, the position of the search bar is above the tabs. Global Search returns personalized results, based on which records a user views most (opportunities are placed at the top if the user views opportunity most frequently).
Global Search will return more search results than sidebar search, as it indexes most custom fields and additional data (including articles, documents, products, and more).
When Chatter is disabled, sidebar search is present. Sidebar search does not return personalized results. It also returns fewer search results than Global Search, as it indexes only select fields (name, address, phone, email, custom fields that are either unique or an external id).
Here’s an account with a unique value populated in custom field “SLA Serial Number”:
Global Search indexes custom fields, therefore search results are returned:
Limitations for Search in Salesforce Classic
[Should / Long / Salesforce.com]
|Menu Above Tabs|
|More Results Returned|
|Menu in Sidebar|
|Fewer Results Returned|