The main differences between the two lie in their manufacturing methods and intended applications:
ASTM A53 Grade B: Suitable for general-purpose piping (fluid conveyance, fire protection, structural applications); may be manufactured using either the ERW or seamless process; not recommended for continuous high-temperature service;
ASTM A106 Grade B: Available only as seamless pipe; specifically designed for high-temperature environments (boilers, heat exchangers, high-temperature process piping).