What is the "session" field in Concourse logs?

[marco-m-pix4d]

Hello,
the majority of concourse web logs have a "session" field. For example:

{
  "timestamp":"2024-03-18T05:17:27.469802205Z",
  "message":"atc.tracker.notify.run.running",
  "data":{
    "build":"2",
    "build_id":12345,
    "job":"my-job",
    "pipeline":"my-pipeline",
    "session":"27.2161.2",  <== HERE
    "team":"my-team"
  }
}

I searched in the Concourse source code but could not find the origin.
I found that the logger used by Concourse, lager, has a feature called Session(), but as I understand it, is a false positive.
I found function func (b *build) LagerData() lager.Data in file atc/db/build.go and I was excited for a moment, but it does not add a "session" field.
I found something in the Concourse code related to a session with the database, but there also I could not find any place where the "session" field is added to lager.
I found that the SSH server implementation used by Concourse has the notion of a session hash, but here also I think it is a false positive.

I noticed that, given a build_id, the session string has the same prefix but the length is different over the various events logged.

I also noticed that the session field repeats itself after a few days, so to me it might encode somehow a network connection plus something else that makes it slightly more unique...

Any hints ? Thanks!