The strength grade mainly refers to carbon steel fasteners.
The common strength grades of carbon steel screws are: grade 4.8, grade 5.8, grade 6.8, grade 8.8, grade 10.9 and grade 12.9. There are four grades, six grades, eight grades, ten grades and twelve grades.
Generally, screws below grade 8.8 are called ordinary screws, while those above grade 8.8 (including grade 8.8) are high strength screws. The difference is that high strength screws need to be quenched and tempered.