Select one of the following authentication options:
Use the authenticated user's account Select this option to use the Windows user name and password of the current user. This is the most secure method, but it can impact performance when there are many users.
Use a stored account Select this option to use Secure Storage Service (SSS). You must enter an Application ID, provided by the site administrator responsible for the SSS instance. A site administrator can configure a SharePoint site to host a Secure Storage Service database where such user names and passwords can be stored and accessed by Excel Services for authentication. This method can be the most efficient when there are many users.
None Select this option to use authentication information saved in the connection string, for example, when you select the Save Password check box.
Note: Avoid saving logon information when connecting to data sources. This information may be stored as plain text, and a malicious user could access the information to compromise the security of the data source.
Note: The authentication setting is used only by Microsoft Excel Services, and not by the Excel desktop program. If you want to ensure that the same data is accessed whether you open the workbook in Excel or Excel Services, make sure that the authentication settings for the connection are the same in Excel and Excel Services.