From one of the resources on the internet, Little Machine Shop, this bit of wisdom:
These Starrett Machinist Levels have ground and graduated main vials, and a cross test vial. The 12" level also has a plumb vial. These vials are adjustable to a positive setting and are housed in a satin chrome brass tube with a friction-fit closing cover to prevent breakage.
The base of the precision machinists level features an involute groove running the length of the base, which provides a reliable seat for round work such as pipes or shafting. With the cross test vial, it is possible to simultaneously level in both directions. This prevents inaccuracies in the main vial reading caused by canting the level sidewise on round work.
The main level vials have graduations that are approximately 80-90 seconds or .005" per foot (0.42 mm per meter). There are five, six, or seven lines on each side of the bubble, depending on the base length.
Starrett Machinist Levels are the finest levels available, used for precision work that is typically required in industry. They all have these features:
All level bases are made from the finest quality seasoned cast iron and are precision-machined on the reference surface. Nonmachined surfaces have an attractive, black wrinkle finish. An involute longitudinal groove between the bearing flats for accurate seating on round work. This groove has a special involute design, permitting better centering and greater capacity to handle larger rounds. Groove and bearing flats are machined together for maximum accuracy.
If you have the bubble centered on the level, it's level within 5 thousandths of an inch over 12 inches, which is astonishing to my alleged mind. One bubble mark off and your reference becomes 10 thousandths of an inch over the same 12 inches.
You asked about the 0.42 mm per meter, which is applied in the same manner. The distance is larger, one meter compared to one foot, and the corresponding precision figure is appropriately larger. I didn't perform the math, but I expect the two numbers are equivalent.
The 80-90 seconds reference is a measurement of degrees. 360 degrees per circle, 60 minutes per degree, 60 seconds per minute. Another astonishingly tiny reference, but machinists often work in tenths of a thousandths of an inch and require such precision.