The good think is that both the mtb and road sets are compatible. It's only really necessary to think in terms of whether you need 18 gears (which is then subdivided into road or a compact spacing) or whether you need the greater range afforded by having 27 gears in a triple configuration.
For example a tourer might want 27 gears to cover every variation of hill and load. The actual ratios are best described in terms of 'inches' (bizarrely being the equivalent diameter of the big wheel of a penny farthing!), and for a tourer these might stretch from a 24 inch granny gear, up to a 100 + inch gear for flying along with the wind up your chuff.
To achieve this (and at a reasonable compromise between lightness / durability / cost), you might pick a 12 to 32 tooth rear cassette from the Shimano LX mountain bike range, some bar end gear shifters from the old Shimano Dura ace 9 speed road range, a Ultegra triple front shifter from the road range, an XT rear shifter again from the mtb range. To complement this, you might pick Shimano's touring option of a Deore (Mtb) chainset but with the touring chain rings with 26 / 36 / 48 teeth, or you might choose to skip Shimano altogether, and go for one of the slightly more specialist touring chainset from French firm TA, and pick a wider 24 / 36 / 50 option.
There's also plenty of people who might also substitute a SRAM rear cassette and chain into that mix and Belgian made 'Frogs Leg' cantilever brakes.
No manufacturer makes the perfect touring set these days as the market is too small. But by mixing the road and mtb set from Shimano, plus any other manufacturers who offer specialist bits of kit, you can probably find YOUR ideal groupset.
Unfortunately, it's not the cheapest way of speccing a bike. Therefore you will see tourers offered with essentially complete Shimano Deore mtb groupsets, as this is the cheapest way of getting something that is 'good enough', if not perfect.