Section letter assignments are static and assigned for standard publications and are required for specific handling through scripts in the workflow. These should only ever be used on the products they are assigned.
Special sections can use any of the letters listed for the generic zones at right, except that you cannot use the same section letter for more than one publication on the same insertion date in a site.
For example, a CV MOD1 and a CV TAB1 published on January 1, 2020, BOTH cannot be letter H.