https://ethn.io/37137?ref=11, ref - is variable name, and 11 - value.
Then ethnio will see if the value for such a variable name is listed in answers list for hidden question. If ethnio finds an answer with that name (that's the left part of each answer pair) equal to the value, which is again in this case 11, it will get the associated value for it from the right side of the list. Confusing, we know. Basically it tries to match the parameter to the value. So then this answer will be sent as regular predefined answer and you will see it in recruits list as one of answers.
Btw, there's one variable name that will break our system, and that is "id" so you can't have id=id. Guess it's recursive or something.
Passing Variables Without Labels
If ethnio doesn't find an answer with the same value it will not translate it and will display it as is under recruits. So if we got the request https://ethn.io/37137?ref=other_value5 but the question has no other_value5 answer predefined, then in the recruits list, you'll just see it as other_value5. It makes predefined answers not obligatory so you can assign unique IDs from your database to ethnio responses if you'd like.
Sending a Variable to a Third Party Tool
If you pass something like Visitor ID along to your ethnio screener, and then you'd like to also append that to the URL you send to a tool like Surveymonkey, you can do that! Just check this little box under logic. The syntax will be the same as you set under questions - so if you have it set to "ethn.io/37137?visitorID=" that means we'll send it as https://www.surveymonkey.com/r/5Y45?visitorID=" Of course this means you have to be using logic.