The CODE key is a modifier key, mostly found on typewriters but is also on some computer keyboards. It is used as a third-layer Shift key for accessing special functions and symbols on other keys, thus acting as a Fn key and an Alt Gr key in one. The legend is often capitalised.
On other typewriters (even from the same manufacturer) the corresponding key is sometimes labelled MOD.
Triumph-Adler typewriters with Cherry MX-compatible keycaps have had a CODE or MOD key. The CODE key was included in a replica set of Triumph-Adler Royal Alpha keycaps, replacing the Windows/Command key. Later it found its way into other key sets as well.