The Reference documentation has this line in Section 1.2.2. How it works, where it describes how the Registrastration example works:

When the form is submitted, JSF asks Seam to resolve the variable named user.

I see the code in the faclet, obviously:

<h:panelGrid columns="2">
 Username: <h:inputText value="#{user.username}" required="true"/>
 Real Name: <h:inputText value="#{user.name}" required="true"/>
 Password: <h:inputSecret value="#{user.password}" required="true"/>
 </h:panelGrid>

How does exactly does Seam get asked to resolve the reference to user? Is it because Seam provides an alternative implementation of the EL?