Web Application Scan is new in Vuluerability Manager, I can't find information talk about how and why setting up credential sets, these are many option for that. Where I can how to setup and information I need!
This is documented in the context sensitive help - click on the "?" inside the Credentials Section of the Scan Configuration.
Select this account type to use credentials for a specific asset running a web application.
Type the host name of the server running the web application.
Select this account type to use credentials when either the Web Domain or Web Individual Host credentials do not work.
Web Application URL
Select this account type to use credentials for a specific web application.
The Web Application field requires a URL (example: http://www.hostname.com). To ensure the credentials are applied to the correct asset, the URL in the Web Application field needs to match the URL entered on the Targets page of the scan configuration.
Note: URLs cannot contain special characters (e.g. &).
We can't possibly tell you which to use over a community forum. You can open a Service Request for help.