Hi,

I assume that once people enter on a certain domain name, they will stay on that domain name. If that's the case, then you don't need cookies or session variables. In fact, cookies and session variables are tied to the specific domain that the user is on, so if they swap domains the cookies or session variables from the previous domain won't even be accessible to Joomla.

So, to use the domain to trigger certain templates or menu item removal or other actions, just use the "Domain name(s)" control in the "Environment" section to enter one or more (comma-separated) domain names. If the user is on one of the listed domain names, the rule will succeed and will run the succeed action.

e.g. if you have these 2 domains:
foo.example.com
bar.example.com

... then you could set up 2 Chameleon rules:
In the 1st - enter foo.example.com into the Domain name(s) box, and in the succeed action select the desired template for that domain
In the 2nd - enter bar.example.com into the Domain name(s) box, and in the succeed action select the desired template for that domain

Cheers,
Stephen