1.Call_form() : It will call another form but parent form(calling form) will remains active. User can not navigate between called from and calling form. To go back to the parent form(calling form) user has to exit explicitly from the child from (called form).
CALL_FORM(form_specification, display, switch_menu, query_mode, parameter_list_id);
2.Open_form(): It will call another form with navigating facility between called from(child form) and calling form(parent form).No need to exit form called form to go back to calling form.
OPEN_FORM(form_specification, activate_mode, session_mode, parameter_list_id);
3.New_form(): It will call another form and exit from calling form(parent form) means there will be only one form on screen.
NEW_FORM(form_specification, rollback_mode, query_mode, parameter_list_id);