Just noticed that all the links in the Newsletter email sent to me contain "nlcode" parameter in the querystring, e.g.


Not sure if it's private info so replaced actual details, the format is A|B, where A is some integer (not my user ID here on in the Network profile), and B is probably some hex value.

What is this value and what is it used for?

  • 1
    Well, tracking of course. I guess Stack Overflow can somehow analyze the effectiveness of their newsletters this way. Hopefully, a developer will come along and provide the details. Of course, the newsletter=1 already provides this, but perhaps the code identifies your particular subscription?
    – Glorfindel Mod
    Commented May 24, 2017 at 13:27
  • We could determine if it was private or not by comparing our nlcode's but, unfortunately, I don't actually get the newsletter.
    – Jason C
    Commented May 24, 2017 at 13:31
  • @Glorfindel you have MSE newsletter? Commented May 24, 2017 at 13:32

1 Answer 1


nlcode stands for newsletter code (nl - news-letter) - see how that works?

The code itself allows us to figure out what exact newsletter subscription caused a specific visit to the site.

As surmised by the OP, this allows us to track newsletter effectiveness, performance and to run A/B tests and experiments on those in order to improve them.

As I've been asked about the format - which takes an a|b form - the | is simply a delimiter, the a stands for the subscription id and the b is its HMAC (because we don't trust people and email providers to not mess around with a url... we always verify).


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