The server cannot store the data needed to complete the request.
507 Insufficient Storage is a WebDAV code that some non-WebDAV APIs adopt for "out of disk space" or "quota exceeded" scenarios. It is more specific than 500 (the server is broken) or 503 (the service is unavailable): 507 says "the storage layer specifically cannot accept this".
When servers should return it: Return 507 when storage is full or quota is exceeded.
HTTP/2 507
content-type: application/json
{"error":"quota exceeded","used_gb":100,"quota_gb":100}
For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.