Keys only become unknown when they are explicitly removed from a session and any license release message is acknowledged.Key Systems usually require a block of initialization data containing information about the stream to be decrypted before they can construct a license request message.A Key System is a generic term for a decryption mechanism and/or content protection provider.Key System strings provide unique identification of a Key System.

Unless otherwise stated, key refers to a decryption key that can be used to decrypt blocks within media data. A key is associated with the session used to provide it to the CDM.They are used by the user agent to select a CDM and identify the source of a key-related event.User agents Within a given system ("somesystem" in the example), subsystems may be defined as determined by the key system provider.(The same key may be present in multiple sessions.) Such keys encrypt each set of stream(s) that requires enforcement of a meaningfully different policy with a distinct key (and key ID).For example, if policies may differ between two video resolutions, stream(s) containing one resolution should not be encrypted with the key used to encrypt stream(s) containing the other resolution.