It’s a typo in Carbide Copper’s doc. Definitely GERBER_RS274X. I used that to export files from EAGLE (8.3.2) with Carbide’s CAM processor (here) and used that in Carbide Copper and it went fine, with two road bumps: one minor about hole scaling that required tuning the export a bit, and one major which is that the generated G-code does not raise the tool at the beginning of the job, which can easily be fixed manually in the G-code file: see Carbide Copper drill/holes scaling issue
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