Question about HeapTupleHeaderData

Hi,
I'm writing some papers about PostgreSQL physical storage and I'm
bumping into the HeapTupleHeaderData layout as descripted here
http://www.postgresql.org/docs/8.2/static/storage-page-layout.html

Reading the source code in src/include/access/htup.h I got a question.

The t_cmin and t_cmax stores the insert and delete CID stamps.
the CID (command that operate on item id?) is similar to XID? where I
can find some docs about CID and its meaning?

Thank you very much in advance.
Regards
Federico
--
Federico Campoli is:
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