The Rules so far have been concerned with classification at the heading level as applied by Rules 1 to 5. Rule 6 deals with sub-headings that may also apply to the main heading you have identified. This means that your sub-heading (in this case also called a 'two-dash sub-heading') will be six digits, rather than four digits long.
An example of this occurs in the Tariff chapter 71 where the scope assigned to the term 'platinum' in chapter 4 (b) differs from that assigned to 'platinum' in sub-heading note 2, for the purpose of interpreting subheadings (7110.11) and (7110.19).
It also enables you to compare two or more sub-headings that appear to have similar status within the classification system.