Nf4 is a numeric format

NF4 is an alphabet, not a scheme: sixteen four-bit code levels placed where normally-distributed weights actually fall, instead of evenly. In the five-layer taxonomy — numeric format, scheme, method, container, execution — it sits in the first layer, beside BF16, FP8 and NVFP4. VINDEX3 treats any such alphabet as a representation encoding: chosen per component, recorded in the directory, never inferred from a filename.