That grades are according old DIN standards (DIN 267) or ISO 898, but they are just carbon steels defined by mechanical properties rather than chemical composition.

According to above mentioned standard, the first digit is the tensile strength expressed in N/mm² and divided by one hundred. Second digit express the yield strength as percentage of tensile and divided by ten.

Examples:

5.8 means a steel with a tensile strength of 500 N/mm² and a yield strength of 80% the tensile one, that is 400 N/mm² .

5.6 means TS ≥ 500 N/mm² and YS ≥ 300 N/mm²

8.8 means TS ≥ 800 N/mm² and YS ≥ 640 N/m

And so on...

Best regards