CTR refers to the AES-CTR (counter mode) protection scheme of Common Encryption, identified by the four-character code 'cenc'. It was the original scheme used by Widevine and PlayReady, and it is not byte-compatible with the cbcs (AES-CBC) scheme that FairPlay requires.
That incompatibility is the historical reason platforms once had to encrypt and store premium content twice - cenc for Widevine/PlayReady and cbcs for FairPlay. The industry has since converged toward cbcs so a single CMAF copy can serve all three DRMs, but cenc/CTR remains widely deployed and is essential to understand when reasoning about why a stream may not be interoperable across DRM systems.
