Searched docs that I could find. It seems the most authoritative doc on it is under “How does Cloudflare handle HTTP Request headers”
It says “The CF-Ray header is added which includes a hash appended with the data center the request came through.”
As far as I understand, Cloudflare routing from the request edge data center (the ‘closest’ to the client) to the origin may be dependent on a number of factors - particularly in the case that Argo is turned on. So, which data center is appended to the end of the header value? The first data center that accepted the request or the last one before cf connecting to the origin? Something else?