Class A and Class B are designations derived from ASTM indicating the relative tolerances of volumetric ware. ASTM International is an international standards organization, that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services .
They apply to the relative accuracy of the glassware in several areas. These requirements are specified in ASTM E694, "Standard Specification for Volumetric Ware" and its companion document ASTM E542, "Standard Practice for Calibration of Volumetric Ware".
Class A signifies a compliance with applicable construction and accuracy requirements. Class A instruments are the preferred choice for volumetric determinations. Class A flasks are ISO9000 compatible. Certain models listed contain individual serial numbers for added tracability. Usually made from borosilicate glass, these instruments have superior thermal and chemical resistance. The accuracy is also retained over a longer working life span than their Class B counterparts.
Class B flasks are general purpose instruments with the same basic design as Class A. However, volumetric tolerances for Class B instruments are within twice the specified range allowed for Class A, unless otherwise indicated. Soda-lime glass is usually used for these instruments, and thus they are not intended for prolonged exposure to chemicals.
Source: Curtis Matheson Scientific Laboratory Catalogue