An engineers spirit level is generally used to level machines, although they may be used to level large workpieces on machines such as planers. Spirit levels are also used in building construction, by carpenters and masons.
The accuracy of a spirit level can be checked by placing it on any flat surface, marking the bubble's position and rotating the level 180°. The position of the bubble should then be symmetrical to the first reading.
The levels have a "vee" groove machined along the base which enables the level to sit on a round bar while remaining parallel with the bar's axis. They also have a smaller cross level to enable the second axis to be roughly checked or corrected.
While a precision level may be used to check and correct the twist in a machine (or workpiece), its presence does not necessarily need to be corrected.
A machine such as a mill or lathe does not have to be perfectly level to operate correctly but may in fact have a known twist introduced to the machines bed. This twist is often introduced to ensure that a worn lathe turns parallel work, by realigning the bed (that is worn) to the spindle axis (unworn).
Levelling a ships lathe would be pointless due to the nature of the ships base - floating on water. Correcting any twist in the bed however would be essential for accurate work to be reproduced from the lathe.