The MTB bottom bracket is sized to work with 73mm shells, and comes with spacers to adapt to 68mm. On a MTB frame with a 68mm shell, two of the spacers go on the right side and one on the left to locate the chainline for a MTB drive train. You can re arrange them for a road drive train as long as there is crank and chainring clearance.
EDIT. Looks like we were typing at the same time.