When publishing a survey, sometimes you may want to include certain variables in the URL that allow you to note certain things in the database when analyzing responses. For example, you may want to put in a variable such as invited from=website vs invited from=email. The idea would be to include the variable invited from=website in the link that you supply on your website and include the other variable in the link that you supply when inviting people to take the survey via email. By including a variable like this in the URL, it will automatically be added to the responses of the user who took the survey from that particular URL.
Find out more about GET variables: