A PSSH (Protection System Specific Header) box is a small block of DRM-specific data carried inside an encrypted media file (and signaled in the manifest). Each DRM system has its own PSSH containing what that system needs - key identifiers and the information to construct a license request.
PSSH is what makes Common Encryption multi-DRM work in practice: one encrypted file can carry a Widevine PSSH and a PlayReady PSSH side by side, so whichever DRM the device uses finds its own instructions and fetches the right license. Correct PSSH generation at packaging time is a frequent source of multi-DRM playback bugs when it is wrong or missing for a given system.
