this is the definition for resolution - step, pixel, point doesn't mater this is the minimum possible, if your mechanics have 639 steps per inch, this is your resolution, and one step = 1/639 inch, if you want to increase the resolution you have to gear the motor or use a motor with more steps, or micro stepping drive
i think last one is best, because stepper motor are sh...t , and micro stepping improve a lot their performance