The main differences between ASTM A501 and A500 are that A501 provides standards for hot-formed welded and seamless carbon steel structural tubing, covering round, square, and other configurations, while A500 may be more concerned with cold-formed welded and seamless carbon steel structural tubing, and the specific standards may be different.A501 allows for special shapes and sizes, while the specific sizes and shapes for A500 may be detailed in its standard. A501 allows special shapes and sizes, while the specific sizes and shapes of A500 may be specified in its standard.