Where it comes from
You click a story link in a publisher's newsletter and land on the publisher's site with oly_enc_id= on the end of the URL. That value is your Encrypted Customer Id in Omeda, the customer data and email platform behind the newsletter. When a publisher sets up an Olytics domain in Omeda's web behavior tracking, emails sent from Omeda's Email Builder or Odyssey that link to that domain are flagged as Olytics links automatically and get the oly_enc_id parameter and value appended. Publishers who send through another email service can add the same parameter as a merge variable.
What the site does with it
Omeda's Olytics script creates two cookies on the publisher's site. oly_anon_id holds an anonymous record ID for a new visitor, and a cookie also named oly_enc_id holds the Encrypted Customer Id once the visitor is known. Omeda says the most common way an anonymous visitor becomes known is "the user clicking on a link that contains the oly_enc_id parameter populated with the user's Encrypted Customer Id." After that, behavior is tied to the known record, and the anonymous history in that browser is merged into it.
It can follow a form, too
Omeda's guide to sending someone back to your site after a subscription or newsletter form suggests adding ?oly_enc_id=%%0.2.110%% to the return URL "in case it is needed," and notes the merge variable "is standard across all databases." So the parameter can also appear just after you submit a signup form. Omeda's setup guide warns publishers that oly_anon_id and oly_enc_id will start appearing on the URLs in their Google Analytics reports.
Before you forward the link
The article is the same without it, but the site uses it to decide who you are. The link carries your ID, not the clicker's, so a forwarded or reposted newsletter link can tie someone else's reading to your record on the publisher's site. Take oly_enc_id off before sharing. Other email platforms do this job with their own parameters, _kx for Klaviyo and mc_eid for Mailchimp.