What are multixactids?

Hi,
when working on fixing the bug related to vacuum freeze, I found out
that there is something called "MultiXactId".

Searching docs showed that it is mentioned only once, in release notes
to 9.3.2:
http://www.postgresql.org/search/?u=%2Fdocs%2F9.3%2F&q=multixactid

What's more - I found that Peter Eisentraut already once asked about
them, and lack of documentation:
http://postgresql.1045698.n5.nabble.com/MultiXactId-concept-underdocumented-td5766754.html

So, my question is - what are multixactids, what are they used for,
where can I find any documentation/explanation/whatever?

It seems to be related in some way to the relfrozenxid/vacuum bug, but
I can't comprehend the relation without knowing what multixactid
actually is.

Best regards,

depesz

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