I'll take a semi-edu-ma-cated stab at it.
CRW: Cold Rolled Welded
WHR: Welded Hot Rolled
WCR: Welded Cold Rolled
Along thse lines you'll also find:
HREW: Hot Rolled Electric Welded
CREW: Cold Rolled Electric Welded
You can likely find just about variation of the above 5 that doesn't mix hot and cold in the same tube. All of these have a visable seam in their ID and a blueish stripe on their OD that DOM does not. DOM's ID and OD are far more consistant (Closer manufacturing tolerance) due to the way it is produced. It can be argued that DOM also has a seam, but because of the type of cold working done to it, the seam has become invisable. Whether DOM has a seam or not, the point is moot. The additional cold working that DOM recieves is more important for design considerations.
"Teach you all I know and you're still stupid"
-- Howdy Lee