Mongo.Session.update_session

You're seeing just the function update_session, go back to Mongo.Session module for more information.
Link to this function

update_session(pid, doc, opts \\ [])

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Specs

update_session(t(), %{key: BSON.Timestamp.t()}, keyword()) :: BSON.document()

Update the operationTime for causally consistent read commands. There is no need to call this function directly. It is called automatically.