MAX_CACHE_DURATION (Property Type 0x04) is a Track Property.
It is an integer expressing the number of milliseconds an Object can be served from a cache. If present, the relay MUST NOT start forwarding any individual Object received through this subscription or fetch after the specified number of milliseconds has elapsed since the beginning of the Object was received. This means Objects earlier in a multi-object stream will expire earlier than Objects later in the stream. Once Objects have expired from cache, their state becomes unknown, and a relay that handles a downstream request that includes those Objects re-requests them.
If MAX_CACHE_DURATION is not sent by the publisher, the Objects can be cached until implementation constraints cause them to be evicted.