Main Attribute of the Form

There is a Main Attribute checkbox for form attributes. When I check it the attribute appears bold. What is this property for? Is it only to write the object when I click write button of the form? If I have a form where I would like to edit 2 objects at a time (a User and his Individual for example) which one should I mark as a main attribute?

Anybody?

"You can assign the Main Form Attribute, i.e. the attribute that defines the form’s standard functionality (the form’s extension).
Important! Please keep in mind a form cannot have more than one main attribute.
Form extension describes additional properties, methods and parameters for a ManagedForm object form; they are determined by the type of the form’s main item."
Developer Guide
It is possible to edit two or more objects in the form. But in this case you should care about conversion object to form data when opening form and backwards when executing Write command. This conversion is automatic only for main attribute. It depends on business logic of your application which attribute you assign the Main.

And what is the recommendation which of objects in the form to mark as main?