The data source is the data source used in the ArcWeb Services Address Finder, Map Image, and Route Finder Services to generate a Locator USA map or route. The default setting is GDT.Streets.US. See ArcWeb Online for a list of valid data sources.
If you change your data source, you need to update it in the web.config file.
To update the data source used for Locator USA:
The file is located in the Web App directory of the destination folder specified during Locator Application installation, for example, C:\Inetpub\wwwroot\LocatorApplication1\Web App.
Find the line
<add key="DATASOURCE" value="GDT.Streets.US" />
and change the value to an alternative data source.
Save the file.