Newall scales are based on inductive scanning principle.
If precision is critical for the application, then optical glass scales are number 1 choice. They provide exceptional accuracy and temperature stability. Draw back of optical linear encoders is that they cannot be used in submerged environments. Correctly installed, they work fine on most metal cutting machines. However if flood coolant is used and the lip seal side of the scale cannot be properly protected from the high pressure sprays, magnetic or inductive options might be a better solution.
Glass scales have lines at 20 micron pitch which translates to 5um native resolution. Magnetic and inductive scales, depending on the model, have pitch between 1mm to 6mm, and then signal gets approximated 100+ times to get down to 5 microns resolution. With approximation come errors. Exposed magnetic scales also attract fine metal chips, which further degrade accuracy and cause random glitches. Inductive scales, if not sealed, could be similarly affected by fine metal dust (e.g. cast iron) and certain EMI frequencies.
P.S. Higher end glass scales typically have air inlets. Using those creates a positive internal pressure inside of the scale and increases protection level from standard IP53 to IP64.