What kinds of German messages were encrypted by the Tunny machine?

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The Colossus computer at Bletchley Park, Buckinghamshire, England, c. 1943. — Colossus Funding for this code-breaking machine came from the Ultra project.

The Tunny machine, a sophisticated 12-wheel cipher device used by the German military, was reserved for the highest grade of military intelligence. Unlike the Enigma machine, which handled a wide range of communications, Tunny was specifically used for messages exchanged between the German army’s high command and generals in the field.1

These decrypted messages provided the Allies with detailed insights into German strategy. Most notably, Tunny intercepts revealed German counter-preparations for the 1944 Allied invasion of northern France, known as the Normandy landings.1