question of subform ID

Hi,

     I have a question about subform ID,

     In the CRM sample application, it set the subform ID as "account" same as "Parent Field to keep Subform ID", but if we set subform ID as any other letter, when we create new account in "Opportunity Form" form, this ID will not be created as a table field.

     It looks like if "Parent Field to keep Subform ID" has any ID, it will ignore the subform ID ( not create table field by subform ID), is it right?
     It is different thinking from what I thought the function for subform before.