The difference between the HREW (hot rolled electric welded) and DOM (drawn over mandrel) is strength.

The HREW is basically mild steel with a yield strength of around 40ksi.

The DOM is the same material put through a secondary process where the tube is cold drawn over a mandrel which produces a more consistent and stronger tube. This process creates a tube that has a yield strength of around 70ksi.

The DOM is simply stronger, can take more punishment and is best suited for use against rocks.