What is srsltid?

Who adds it

Google Merchant Center adds it when the merchant has auto-tagging on, to clicks on the merchant's free listings in Google. Google calls it a result id.

What it means

Someone clicked an unpaid Google listing for the merchant, such as a product result. Google uses it to report key events (conversions) from those clicks in Merchant Center.

What to do with it

It's created at impression time, so repeat clicks on the same listing share one value. Merchants can turn it off in Merchant Center's key event setup; a canonical tag keeps the tagged URLs from being treated as separate pages.

How Vireo shows it

Vireo doesn't store click IDs, in the free plugin or in Pro.

In the free plugin, Vireo drops the query string from page addresses, keeping only a few WordPress ones such as p and page_id, so a tagged URL counts as the same page as the untagged one. The visit is counted under the referrer it arrived with.

See Referrers and sources in the docs.

Sources

If someone clicks on your free listings from www.example.com, the final URL will look like www.example.com/?srsltid=123xyz.

Set up key event tracking in Merchant Center - Google Merchant Center Help

The result id is created at the time of an impression.

Set up key event tracking in Merchant Center - Google Merchant Center Help

How to Specify a Canonical with rel="canonical" and Other Methods - Google Search Central