Linear Encoder question

I'm working on a project for which I want to use magnetic linear encoders. I'm seeing lots of options for reader heads out there advertising resolution in the range of 0.025 mm but they ask for tape w ith poles spaced at 2 mm. How does this work? Are there multiple sensors in a single head - spaced appropriately to discern 0.025 mm from 2 mm poles? Does the standard A/B grey code from rotary encoders apply?