What differentiates a tool room lathe from something like a SM 2000 series UtiLathe?
There is a DIN standard irrc, but more I think its just part of the machining vernacular so is somewhat subjective. No expense spared, everything as solid as it can be, belt driven spindle, extended range of speeds, very heavy for the size, often a matched cast iron stand on three points (no possibility of twist), and so on. Examples would be a Monarch 10ee, Hardinge HLV, Schaublin 135, Smart Brown 1024, Rivett 1020, Smart Brown 1024, Hendy T&G and so forth
On my 10ee for example, its a 10x20" lathe, 3 HP, 3000 pounds, later models had 10-4000 rpm, belt driven spindle, saddle so massive parts of the bed never see daylight, massive cast iron stand matched to the bed, built in pump oiler for bed ways and cross slide and so on. When I redid the feed/thread gearbox and apron, I probably replaced 30 or 40 rolling element bearings, just everything was done to the max. All quite different from a comparable SM, which I emphasize is a great and capable lathe, but just a not in the category.