203 NON-AUTHORITATIVE INFORMATION
The request was successful but the enclosed payload has been modified from that of the origin server’s 200 OK response by a transforming proxy.
This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might impact later decisions regarding the content. For example, future cache validation requests for the content might only be applicable along the same request path (through the same proxies).
The 203 response is similar to the Warning code of 214 Transformation Applied, which has the advantage of being applicable to responses with any status code.
A 203 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls.
Transformations RFC7230 Section 5.7.2
Warning RFC7234 Section 5.5
Calculating Heuristic Freshness RFC7234 Section 4.2.2
203 NON-AUTHORITATIVE INFORMATION Source: RFC7231 Section 6.3.4
203 CODE REFERENCES
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant
Python2 HTTP Status Constant
Python3+ HTTP Status Constant
Python3.5+ HTTP Status Constant