hi lenka,
i am not able to get your question properly, mind rephrasing it(if you dont get your answers below).
As far as functionality of optional section is concerned.
1) we can have default value set to fields in optional sections, there value will be available/visible only when the optional section is present on the form.
2) when we remove the optional section from the form, its value is also removed, i.e it is acatually removed from the form not hidden or any thing like that.
3) when submitting the form, if the optional form is removed from the form, the promoted field of optional section will appear blank, if the optional section is visible on the form, then the corresponding values will be promoted.(shown in the sharepoint doc lib).