To optimize wire efficiency, Subgroups and Datagrams refer to a track by a numeric identifier, rather than the Full Track Name. Track Alias is chosen by the publisher and included in SUBSCRIBE_OK (Section 10.8) or PUBLISH (Section 10.10).
The same Track Alias MUST NOT be used by a publisher to refer to two different
Tracks simultaneously in the same session. If a subscriber receives a
PUBLISH or SUBSCRIBE_OK that uses the same Track Alias as a different Track
with an Established subscription, it MUST close the session with error
DUPLICATE_TRACK_ALIAS.
Objects can arrive after a subscription has been cancelled. Subscribers SHOULD retain sufficient state to quickly discard these unwanted Objects, rather than treating them as belonging to an unknown Track Alias.