When the readtable case is :upcase, unescaped constituent characters are converted to uppercase, as specified in Section 2.2 Reader Algorithm.
When the readtable case is :downcase, unescaped constituent characters are converted to lowercase.
When the readtable case is :preserve, the case of all characters remains unchanged.
When the readtable case is :invert, then if all of the unescaped letters in the extended token are of the same case, those (unescaped) letters are converted to the opposite case.
23.1.2.1 Examples of Effect of Readtable Case on the Lisp Reader
23 Reader