Here is some valuable information dealing with types of Tubing, including structures, specifications, applications, size ranges, and chemical analysis.
*Credit goes to 'Old Scout', thanks for the info.*
DOM, ERW, HREW...all types of "mechanical tubing", as opposed to pipe. All mechanical tubing is dimensioned by OD and a wall thickness.
DOM is actually not a type of tubing, but a process that is applied to tubing after it is initially constructed. It is Drawn Over a Mandrel...which "cold works" it, giving more exact dimensions, smoother finish, and better alignment of the crystal latice structure. It is Not seamless tubing, and it started life as some sort of EW (electric welded) tubing...