There is no "grade" for sintered stone in the same way as quartz because sintered stone is 100% mineral (compare to quartz where grade usually has relationship with the ratio of minerals to resin). Sintered stone is functionally a slab of finely ground minerals melted/fused together by high heat and pressure, like a real stone. Any "grade" seen on sintered stone is mostly marketing & branding for show.

Sintered stone is much more heat resistant and scratch resistant than quartz, and the patterns will not fade easily + won't turn yellow because no resin involved in creation of sintered stone. However more pricy than quartz and also more difficult to repair if cracked/chipped.