What links customer to their own information in the portal ?

What links customer to their own information in the portal ?

I know how to give permissions to some parts of my application, and how send portal permission link to my customers, but what exacly links my customers to their own information in the portal ?

I though it was as simple a recognising the email adress in the customer form, but it doesn't seem to be that easy. Is there a perticular setting I have to do ?

Sylvain