I’ve noticed some HTTP Headers prefixed with “x-” but never bothered to look up what this convention meant until now. I figured it was something about “external” or to denote a header concerning more than one application/endpoint. For instance, X-Forwarded-For, which is typically used to communicate the original client IP address before a request is forwarded by a reverse proxy.
This article was an interesting source: https://inspiredmonks.com/what-does-x-mean-in-http-headers/
This unofficially stands for “experimental” or “extension” and was originally supposed to be used for anything which wasn’t a standard HTTP header, like Content-Length.
The “X-” prefix was deprecated by IETF in RFC 6648 (June 2012), but this doesn’t mandate anything about new or existing protocols.
Makes no recommendation as to whether existing “X-” parameters ought to remain in use or be migrated to a format without the “X-”; this is a matter for the creators or maintainers of those parameters.
Somewhat unrelated, I found it very satisfying that certain keywords are used consistently in RFCs and each have their own definitions.