When used in a call to requestMediaKeySystemAccess(), the returned object must support this feature. When returned by a MediaKeySystemAccess object, CDM instances created by the object may use this feature.
optional:
When used in a call to requestMediaKeySystemAccess(), the returned object may support and use this feature. When returned by a MediaKeySystemAccess object, this value cannot and must not be present in such an object.
not-allowed:
When used in a call to requestMediaKeySystemAccess(), the returned object must function without using this feature and must not use it at any time. When returned by a MediaKeySystemAccess object CDM instances created by the object must not use this feature.