The ISO 898-1 strength classification for metric steel fasteners.
The number before the point is one tenth of the nominal tensile strength in MPa; the number after is the ratio of yield to tensile strength times ten. So a class 8.8 bolt has 800 MPa minimum tensile and yields at 80% of that — 640 MPa.
Higher classes are stronger but less ductile, and they are more sensitive to hydrogen embrittlement after plating, which is why 12.9 bolts are usually supplied with a protective coating rather than electroplated.
Substituting a higher property class without checking the joint. A stronger bolt changes the failure mode from bolt failure to thread stripping in the nut.
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