Explanation/Reference:
To configure Excel Services data access to use embedded data connections, you use the following process:
Configure a data access account
(A) Create a Secure Store target application
(B) Configure a workbook to use an embedded data connection
A: You must create a target application in Secure Store that contains the credentials that you created for data access. This target application can then be specified in data-connected Excel workbooks and will be used by Excel Services when it refreshes data in the workbook.
B: Configure a workbook to use an embedded data connection You must configure the Excel Services Authentication Settings in the workbook before you publish it to SharePoint Server 2013. Doing so enables the workbook to use a Secure Store target application to refresh data that is rendered using Excel Services. Use the following procedure to configure the authentication settings.
Note:
* Excel Services in SharePoint Server 2013 provides three methods of using Secure Store Service to refresh the external data source in a workbook:
/You can use an unattended service account.
/You can specify a Secure Store target application in a workbook. (This is known as an embedded connection.)
/You can use an Office Data Connection (ODC) file that specifies a Secure Store target application. This article describes how to do this.
Reference: Configure Excel Services data refresh by using external data connections in SharePoint Server
2013