I'm a bit confusing about the channel variables that represent the call ID.

I find that there are three variables, 1) Unique-ID 2) Channel-Call-UUID 3) variable_sip_call_id, what is the different between this.

The value of Channel-Call-UUID seems to be stable during the entire call, the value of Unique-ID in the begin it is the same as Channel-Call-UUID but when bridging it changes it value to another ID, then it comes back to the beginning value, i.m not clear when and why.

The variable variable_sip_call_id many time don't have any value, and also it changes the value when bridging and then comes back to the previous value.

I have looked for an explanation in FreeSWITCH wiki, but don't find.

In the mailing list a have found an explanation here: http://lists.freeswitch.org/pipermail/freeswitch-users/2013-August/099035.html, but still not enough.
Can someone give a good explanation about this variables,what is their purpose, and so on.