What does the letter B in the bolt BM64*370 stand for?

That’s what it says on the list of bolts that arrived

In which specification or standard did you see that?

Bolts are divided into precision bolts (grades A and B) and rough bolts (grade C). Based on the machining precision of the bolts, ordinary bolts are divided into precision (semi-precision) bolts (grades A and B) and coarse bolts (grade C). Grade C bolts have rough machining and imprecise dimensions; only Class II holes are required (holes that are formed in a single operation on a part or whose diameter is achieved without the use of a drilling die), and they are cost-effective, with the bolt diameter being 1.5–2.0 mm smaller than the hole diameter. Grade A and Grade B bolts must be machined on machine tools; they require high precision and use Class I holes. The diameter of these holes is equal to that of the bolts, with only positive and negative tolerances allowed for each. As a result, the gap between the bolt shaft and the hole is only about 0.3 mm. It can be seen that grade A and grade B bolts and screw holes have a tight fit, resulting in good shear performance and minimal deformation; however, their manufacturing and installation are labor-intensive and they are expensive, which is why they are used less frequently in steel structures at present. Due to the larger gap between them and the bolt holes, grade C bolts still maintain good performance in withstanding tensile forces. Therefore, they are widely used in connections that require disassembly, in installations subject to tensile loads, in less critical connections, or as temporary fixations during installation. ” This is what I copied. You can take a look at the drawings to see which standard the bolts follow; download that standard and check it out