You can use forms based authentication when you wish to authenticate users against an identity management system that isn’t based on Windows. You can also use it for an external system as long as you register the membership provider and role manager in the Web.config file. SharePoint 2013 uses the standard ASP.NET role manager interface in order to collect group information about the current user.
The ASP.NET role is considered to be a domain group by the authorization process in SharePoint 2013. Role mangers are registered in the Web.config file the same way that you would register membership providers for authentication. To manage membership roles from the Central Administration (WCAM) website, you need to register the membership provider and the role manager in Web.config file for the Central Administration website. You also need to register that membership provider and the role manager in the Web.config file for the web application that hosts content.