It's how they name classes of models, presumably this implies something about the relative quantization / size of model, not about the specific performance. E.g

It's how they name classes of models, presumably this implies something about the relative quantization / size of model, not about the specific performance. E.g. Fabel 5 will be better than Opus 5, better than Sonnet 5, etc. The 5 is the version number of the particular iteration / training run at this class of model.