Grades

They are available in various grades depending on their intended use.

reference (AAA): small tolerance (± 0.00005 mm or 0.000002 in) used to establish standards
calibration (AA): (tolerance +0.00010 mm to -0.00005 mm) used to calibrate inspection blocks and very high precision gauging
inspection (A): (tolerance +0.00015 mm to -0.00005 mm) used as toolroom standards for setting other gauging tools
workshop (B): large tolerance (tolerance +0.00025 mm to -0.00015 mm) used as shop standards for precision measurement
More recent grade designations include (U.S. Federal Specification GGG-G-15C):

0.5 — generally equivalent to grade AAA
1 — generally equivalent to grade AA
2 — generally equivalent to grade A+
3 — compromise grade between A and B
and ANSI/ASME B89.1.9M, which defines both absolute deviations from nominal dimensions and parallelism limits as criteria for grade determination. Generally, grades are equivalent to former U.S. Federal grades as follows:

00 — generally equivalent to grade 1 (most exacting flatness and accuracy requirements)
0 — generally equivalent to grade 2
AS-1 — generally equivalent to grade 3 (reportedly stands for American Standard - 1)
AS-2 — generally less accurate than grade 3
K — generally equivalent to grade 00 flatness (parallelism) with grade AS-1 accuracy
The ANSI/ASME standard follows a similar philosophy as set forth in ISO 3650. See the NIST reference below for more detailed information on tolerances for each grade and block size.