Users not only have to install CAYIN MSP but also need to obtain and activate a license (basic is free). Also the old CodexPack is no longer supported from QTS 4.5.1 onward - CAYIN MSP seems to be the replacement app. It’s used for thumbnail generation and transcoding by the multimedia console.
PS: While QTS naged users for quite a while to install CAYIN MSP, QNAP somehow failed to inform the users about the reason behind it IMHO. So will future devices and new installs have CAYIN MSP preinstalled by default?