Cold Rolled Electric Weld (CREW) tubing is considered mild steel as is DOM, but it is not the same. The welded seam tubing is not very accurate as far as tolerances are concerned. I have seen .120 wall stuff as thin as .113 or .114. This is the reason places who use this tubing go to .134 wall. That will sometimes run to .128 to .129 thick, still legal for cages and bars. It is also heavy.
DOM is Drawn Over Mandrel and is much more accurate. You can use .120 wall material and still satisfy the .118 minimum thickness requirement. Also, the DOM I have seen has all been 1020 steel where the welded stuff has all been 1010. This means the DOM has a higher carbon content which makes the steel harder. This makes for a stiffer cage assembly. I can tell the difference running a drill bit through it.
Both of these are considered mild steel, but the DOM is higher quality material and as a result is more expensive than CREW or ERW. When you talk to someone about mild steel tubing, make sure you know if it is welded seam or DOM, which is seamless. Personally, I wouldn't use welded seam tubing for anything on a race car.