Integer or UUID
The cid is the equivalent of the second value in the _utma cookie when you use the javascript tracking. In js tracking, it is a random integer (generated by Math.round(2147483647 * Math.random())). But it is strored and sent as a string : so you can use both formats (integer or UUID).
Required/Optional
With js tracking, a request sent with a missing user id is ignored by Analytics servers. So assume that it is required.
Link web visitor with measurement protocol actions
If you want to link your backend user with a visitor previously tracked with Analytics javascript ga.js, you can get the cid value from the _utma cookie. But I don’t recommend it; each time it changes (terminal or browser change, cookies cleaning, etc.), you will lose the link with your customer’s previous actions.
Update
The analytics.js did not exist when the question was asked. cid is now stored in the cookie _ga. You can get it in javascript with:
ga(function(tracker) {
var clientId = tracker.get('clientId');
});
As of writing this, when you do get the clientId from the tracker as in the above code it won’t give you a UUID but two random 32-bit integers in the format of “X.Y”. This implementation may change to the UUID in the future.
Or set it with your own value (like a backend generated UUID):
ga('create', 'UA-XXXXX-Y', {'clientId': 'your_client_id'});