where is the variable set?

The form when the submit button is hit cfinvokes the security.cfc passing it arguments made from form fields (form not shown for brevity). The cfinvoke sets(is that the right way to say it?) a return variable called authenticated from the authenticated method of the security.cfc

It works fine but I can´t donderstand why if the cfreturn tag sends back checkAuthentication.username why returnVariable="authenticated" works? Thanks for any explanation