"A grimoire is a textbook of magic and sorcery. Traditionally, it contains instructions for casting spells, performing divination, creating magical objects like talismans, and summoning supernatural entities such as angels or spirits."
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.