It seems that the Rank field can not be configured when creating a new Context for the JIRA Agile Classic Board according to the documentation. Please refer to the following screenshot(s):

• Try to create a new Context as per the following screenshot:
  

• As opposed to the Screenshot: Sorting the Context by the Ranking custom field in the documentation, we are not able to see the Rank field as expected:
  

Is this an expected behaviour or is there any configuration missing? The same behavior is also observed when creating Project Template as well.